LINQ Expression Trees-Lambdas to CodeDom Conversion
Introduction
Some people are working to make the meta-programming possible. Some says as language oriented programming or domain specific language, but I prefer in general as meta-programming. For years programming languages supported to generate code with the powerful libraries or developers worked just with string concatenations and external linkers.
Nowadays meta-programming is getting more and more important as the domain expertise required. So the languages make meta programming possible at the compiler level with compiler directives.
Indeed there a lot of ideas coming from functional programming world where everything treated as expressions. The code becomes data and data usage happens in the code. It should sound familiar with LINQ to SQL efforts to make this possible.
Libraries
.NET Framework had code generators since the beginning. CodeDom is probably the best known for tree based code generation. Codedom made possible to develop the ASP.NET engine, Windows Form designer, Web form designer, Web services wrapper LINQ entity objects and more. It is used extensively by the framework for the key technologies.
Although there are other APIs in .NET framework such as System.Reflection, System.Reflection.Emit, in this post we will focus on CodeDom and the new comer Expression Trees.
Expression Tree is the key API behind LINQ to SQL or IQueryable interface in general. Every query is expressed as typed trees that is parsed and converted to SQL later by the library.
The syntax of expressing queries is very readable with query comprehension syntax. However sometimes I want to know about the generated tree, like actually which functions are getting involved in the query. I have used Expression Tree Debugger Visualizer to draw the tree. It is pretty handy tool but for big trees it is difficult to see what is going on. This was my main motivation actually, although we had the code, we don’t see what’s the magic going on with query comprehension.
Implementation
So the idea is to have the code regenerated from the tree. In the real world this will involve a parser, interpreter and some more compiler theory which requires a lot of research. And because this is just for fun and since we have a powerful CodeDom library to generate code, I tried to convert the expression tree to CodeDom tree. Than used the CodeDom to generate code in any language. Finally I wrote the extension methods so that the debuggers and my code can use it directly from the type.
The compiler generates automatically the expression trees if we use the proper syntax. So from the beginning we have the tree. In order to convert to CodeDom objects, we need to traverse the tree and generate the necessary CodeDom objects. So I wrote a tree walker that generates a CodeDom object to is parent while going to the last children. I didn’t realise how far it is going but that was it. When the tree walker finished with some more few lines of code the converter was just working.
I would like to put the code here as well but unfortunately it is too long for a blog post, so here are some snippets. Feel free to provide suggestions or bug reports.
LINQ Expression Visitor that generates CodeDom Trees
using System; using System.Collections.Generic; using System.Linq; using System.Text; using System.Collections.ObjectModel; using System.Linq.Expressions; using System.Reflection; using System.CodeDom; using System.CodeDom.Compiler; using System.IO; namespace ExpressionToCodedom { public class CodeDomExpressionVisitor { Expression m_exp; Dictionary<string, CodeTypeMember> m_members; public CodeDomExpressionVisitor(Expression e) { m_exp = e; } internal string GenerateSource(CodeDomProvider codeProvider) { StringBuilder sb = new StringBuilder(); TextWriter tWriter = new IndentedTextWriter(new StringWriter(sb)); CodeCompileUnit ccu = GenerateCode(); codeProvider.GenerateCodeFromCompileUnit(ccu, tWriter, new CodeGeneratorOptions()); codeProvider.Dispose(); tWriter.Close(); return sb.ToString(); } internal string GenerateSource(string language) { CodeDomProvider codeProvider=null; if (language == "cs") codeProvider = new Microsoft.CSharp.CSharpCodeProvider(); else if (language == "vb") codeProvider = new Microsoft.VisualBasic.VBCodeProvider(); else { throw new Exception("make sure you are trying to load a CodeDomProvider assembly"); } return GenerateSource(codeProvider); } public string GenerateSource() { return GenerateSource("cs"); } private CodeCompileUnit GenerateCode() { var code = new CodeCompileUnit(); m_members = new Dictionary<string, CodeTypeMember>(); var LambdaTypeClass = new CodeTypeDeclaration("LambdaExpression"); var ns = new CodeNamespace("Runtime"); ns.Types.Add(LambdaTypeClass); ns.Imports.Add(new CodeNamespaceImport("System")); // add more types in case I want to compile code.Namespaces.Add(ns); CodeObject cEvaluationResult = Visit(m_exp); var constructor = new CodeConstructor(); if (cEvaluationResult is CodeStatement) constructor.Statements.Add(cEvaluationResult as CodeStatement); else if (cEvaluationResult is CodeExpression) constructor.Statements.Add(cEvaluationResult as CodeExpression); LambdaTypeClass.Members.Add(constructor); foreach (var item in m_members) { LambdaTypeClass.Members.Add(item.Value); } return code; } protected virtual CodeObject Visit(Expression exp) { if (exp == null) return null; switch (exp.NodeType) { case ExpressionType.Negate: case ExpressionType.NegateChecked: case ExpressionType.Not: case ExpressionType.Convert: case ExpressionType.ConvertChecked: case ExpressionType.ArrayLength: case ExpressionType.Quote: case ExpressionType.TypeAs: return this.VisitUnary((UnaryExpression)exp); case ExpressionType.Add: case ExpressionType.AddChecked: case ExpressionType.Subtract: case ExpressionType.SubtractChecked: case ExpressionType.Multiply: case ExpressionType.MultiplyChecked: case ExpressionType.Divide: case ExpressionType.Modulo: case ExpressionType.And: case ExpressionType.AndAlso: case ExpressionType.Or: case ExpressionType.OrElse: case ExpressionType.LessThan: case ExpressionType.LessThanOrEqual: case ExpressionType.GreaterThan: case ExpressionType.GreaterThanOrEqual: case ExpressionType.Equal: case ExpressionType.NotEqual: case ExpressionType.Coalesce: case ExpressionType.ArrayIndex: case ExpressionType.RightShift: case ExpressionType.LeftShift: case ExpressionType.ExclusiveOr: return this.VisitBinary((BinaryExpression)exp); case ExpressionType.TypeIs: return this.VisitTypeIs((TypeBinaryExpression)exp); case ExpressionType.Conditional: return this.VisitConditional((ConditionalExpression)exp); case ExpressionType.Constant: return this.VisitConstant((ConstantExpression)exp); case ExpressionType.Parameter: return this.VisitParameter((ParameterExpression)exp); case ExpressionType.MemberAccess: return this.VisitMemberAccess((MemberExpression)exp); case ExpressionType.Call: return this.VisitMethodCall((MethodCallExpression)exp); case ExpressionType.Lambda: return this.VisitLambda((LambdaExpression)exp); case ExpressionType.New: return this.VisitNew((NewExpression)exp); case ExpressionType.NewArrayInit: case ExpressionType.NewArrayBounds: return this.VisitNewArray((NewArrayExpression)exp); case ExpressionType.Invoke: return this.VisitInvocation((InvocationExpression)exp); case ExpressionType.MemberInit: return this.VisitMemberInit((MemberInitExpression)exp); case ExpressionType.ListInit: return this.VisitListInit((ListInitExpression)exp); default: throw new Exception(string.Format("Unhandled expression type: '{0}'", exp.NodeType)); } } protected virtual CodeObject VisitBinding(MemberBinding binding) { switch (binding.BindingType) { case MemberBindingType.Assignment: return this.VisitMemberAssignment((MemberAssignment)binding); case MemberBindingType.MemberBinding: return this.VisitMemberMemberBinding((MemberMemberBinding)binding); case MemberBindingType.ListBinding: return this.VisitMemberListBinding((MemberListBinding)binding); default: throw new Exception(string.Format("Unhandled binding type '{0}'", binding.BindingType)); } } protected virtual CodeExpression VisitElementInitializer(ElementInit initializer) { ReadOnlyCollection<CodeExpression> arguments = this.VisitExpressionList(initializer.Arguments); return new CodeMethodInvokeExpression(new CodeMethodReferenceExpression(new CodeThisReferenceExpression(),initializer.AddMethod.Name), arguments.ToArray()); } protected virtual CodeObject VisitUnary(UnaryExpression u) { CodeObject operand = this.Visit(u.Operand); return operand; } private CodeBinaryOperatorType BindOperant(ExpressionType e) { switch (e) { case ExpressionType.Add: case ExpressionType.AddChecked: return CodeBinaryOperatorType.Add; case ExpressionType.And: return CodeBinaryOperatorType.BitwiseAnd; case ExpressionType.AndAlso: return CodeBinaryOperatorType.BooleanAnd; case ExpressionType.Or: return CodeBinaryOperatorType.BitwiseOr; case ExpressionType.OrElse: return CodeBinaryOperatorType.BooleanOr; case ExpressionType.ExclusiveOr: case ExpressionType.ArrayIndex: case ExpressionType.Coalesce: case ExpressionType.RightShift: case ExpressionType.LeftShift: throw new NotSupportedException("no direct equivalent in codedom,so workarounds not implemented"); case ExpressionType.Equal: return CodeBinaryOperatorType.IdentityEquality; case ExpressionType.NotEqual: return CodeBinaryOperatorType.IdentityInequality; case ExpressionType.GreaterThan: return CodeBinaryOperatorType.GreaterThan; case ExpressionType.GreaterThanOrEqual: return CodeBinaryOperatorType.GreaterThanOrEqual; case ExpressionType.LessThan: return CodeBinaryOperatorType.LessThan; case ExpressionType.LessThanOrEqual: return CodeBinaryOperatorType.LessThanOrEqual; case ExpressionType.Multiply: case ExpressionType.MultiplyChecked: return CodeBinaryOperatorType.Multiply; case ExpressionType.Subtract: case ExpressionType.SubtractChecked: return CodeBinaryOperatorType.Subtract; case ExpressionType.Power: case ExpressionType.Divide: return CodeBinaryOperatorType.Divide; case ExpressionType.Modulo: return CodeBinaryOperatorType.Modulus; default: throw new Exception("are you sure you are right?"); } } protected virtual CodeBinaryOperatorExpression VisitBinary(BinaryExpression b) { var left = this.Visit(b.Left) as CodeExpression; var right = this.Visit(b.Right) as CodeExpression; CodeObject conversion = this.Visit(b.Conversion); CodeBinaryOperatorType operant = BindOperant(b.NodeType); var condExpr = new CodeBinaryOperatorExpression(left, operant, right); return condExpr; } protected virtual CodeObject VisitTypeIs(TypeBinaryExpression b) { CodeObject expr = this.Visit(b.Expression); return expr; } protected virtual CodeExpression VisitConstant(ConstantExpression c) { if (c.Value == null) { return new CodePrimitiveExpression(null); } else if (c.Value.GetType().IsValueType || c.Value.GetType() == typeof(string)) { return new CodePrimitiveExpression(c.Value); } else { return new CodeVariableReferenceExpression(c.Value.ToString()); } } protected virtual CodeObject VisitConditional(ConditionalExpression c) { CodeObject test = this.Visit(c.Test); CodeExpression ifTrue = this.Visit(c.IfTrue) as CodeExpression; CodeExpression ifFalse = this.Visit(c.IfFalse) as CodeExpression; var ifStatement = new CodeConditionStatement(test as CodeExpression, new CodeStatement[] {new CodeExpressionStatement(ifTrue) }, new CodeStatement[] {new CodeExpressionStatement(ifFalse) }); return ifStatement; } protected virtual CodeObject VisitParameter(ParameterExpression p) { return new CodeArgumentReferenceExpression(p.Name); } protected virtual CodeObject VisitMemberAccess(MemberExpression m) { CodeObject exp = this.Visit(m.Expression); if (exp is CodePrimitiveExpression) { return exp; } else { Type memType; if (m.Member.MemberType == MemberTypes.Field) memType = (m.Member as FieldInfo).FieldType; else memType = (m.Member as PropertyInfo).PropertyType; m_members[m.Member.Name] = new CodeMemberField(memType, m.Member.Name); return new CodeVariableReferenceExpression(m.Member.Name); } } protected virtual CodeObject VisitMethodCall(MethodCallExpression m) { CodeObject obj = this.Visit(m.Object); IEnumerable<CodeExpression> args = this.VisitExpressionList(m.Arguments); if (obj == null) { //static method call return new CodeMethodInvokeExpression(new CodeTypeReferenceExpression(m.Method.DeclaringType),m.Method.Name,args.ToArray()); } else { return new CodeMethodInvokeExpression(obj as CodeExpression, m.Method.Name, args.ToArray()); } } protected virtual ReadOnlyCollection<CodeExpression> VisitExpressionList(ReadOnlyCollection<Expression> original) { List<CodeExpression> list = new List<CodeExpression>(); for (int i = 0, n = original.Count; i < n; i++) { CodeExpression p = (CodeExpression)this.Visit(original[i]); list.Add(p); } return list.AsReadOnly(); } protected virtual CodeExpression VisitMemberAssignment(MemberAssignment assignment) {// thhose are properties CodeObject e = this.Visit(assignment.Expression); return e as CodeExpression; } protected virtual CodeObject VisitMemberMemberBinding(MemberMemberBinding binding) { IEnumerable<CodeExpression> bindings = this.VisitBindingList(binding.Bindings) as IEnumerable<CodeExpression>; return new CodeObjectCreateExpression(binding.Member.Name, bindings.ToArray()); } protected virtual CodeObject VisitMemberListBinding(MemberListBinding binding) { IEnumerable<CodeExpression> initializers = this.VisitElementInitializerList(binding.Initializers); return new CodeObjectCreateExpression(binding.Member.Name, initializers.ToArray()); } protected virtual IEnumerable<CodeExpression> VisitBindingList(ReadOnlyCollection<MemberBinding> original) { List<CodeExpression> list = new List<CodeExpression>(); for (int i = 0, n = original.Count; i < n; i++) { CodeExpression b = this.VisitBinding(original[i]) as CodeExpression; list.Add(b); } return list; } protected virtual IEnumerable<CodeExpression> VisitElementInitializerList(ReadOnlyCollection<ElementInit> original) { List<CodeExpression> list = new List<CodeExpression>(); for (int i = 0, n = original.Count; i < n; i++) { CodeExpression init = this.VisitElementInitializer(original[i]); list.Add(init); } return list; } protected CodeMethodReferenceExpression VisitLambda(LambdaExpression lambda) { var body = this.Visit(lambda.Body); var lambdaMethod = new CodeMemberMethod(); lambdaMethod.Name = lambda.Type.Name; if (lambdaMethod.Name.Contains("Func")) lambdaMethod.ReturnType = new CodeTypeReference(lambda.Body.Type); foreach (var item in lambda.Parameters) { lambdaMethod.Parameters.Add(new CodeParameterDeclarationExpression(item.Type, item.Name)); } if (body is CodeExpression) { if (lambdaMethod.ReturnType.BaseType.Contains("Void")) lambdaMethod.Statements.Add((body as CodeExpression )); else lambdaMethod.Statements.Add(new CodeMethodReturnStatement(body as CodeExpression)); } else if (body is CodeStatement) { lambdaMethod.Statements.Add((body as CodeStatement)); } else { throw new Exception("investigate..."); } m_members[lambda.Type.FullName] = lambdaMethod; return new CodeMethodReferenceExpression(new CodeThisReferenceExpression(), lambdaMethod.Name) ; } protected virtual CodeObject VisitNew(NewExpression nex) { IEnumerable<CodeExpression> args = this.VisitExpressionList(nex.Arguments); return new CodeObjectCreateExpression(nex.Type.Name,args.ToArray()); } protected virtual CodeObject VisitMemberInit(MemberInitExpression init) { CodeObject n = this.VisitNew(init.NewExpression); CodeExpression[] bindings = this.VisitBindingList(init.Bindings).ToArray(); //binding will return property initialisation for (int i = 0; i < init.Bindings.Count; i++) { // need to do something with that//// var assignProperty = new CodeAssignStatement(new CodePropertyReferenceExpression( n as CodeExpression, init.Bindings[i].Member.Name), bindings[i]); } return n; } protected virtual CodeObject VisitListInit(ListInitExpression init) { CodeObject n = this.VisitNew(init.NewExpression); IEnumerable<CodeExpression> initializers = this.VisitElementInitializerList(init.Initializers); return n; } protected virtual CodeObject VisitNewArray(NewArrayExpression na) { IEnumerable<CodeExpression> exprs = this.VisitExpressionList(na.Expressions); return new CodeArrayCreateExpression(new CodeTypeReference(na.Type), exprs.ToArray()); } protected virtual CodeObject VisitInvocation(InvocationExpression iv) { IEnumerable<CodeExpression> args = this.VisitExpressionList(iv.Arguments); var expr = this.Visit(iv.Expression) as CodeExpression; return new CodeMethodInvokeExpression(new CodeMethodReferenceExpression(expr, "Method"), args.ToArray()); } } }
Example
The extension methods enables to see the source code of any IQueryable and any Expression. Any of them have a GenerateSourceCodeMethod that gives back a string.
GenerateSourceCode(); // default C#
GenerateSourceCode(string language); // either cs or vb as input or Fully qualified name of the CodeDomProvider (like Microsoft.FSharp.Compiler.CodeDom.FSharpCodeProvider) It should be added as a reference to the project if you’re going to use it.
Sample program that manipulates the expression trees and usage of CodeDom Converter with “item.GetCodeDomSource(“vb”)”
int a = 3, c = 2, d = 0; var e1 = Expression.Constant(5); var e2 = Expression.And(e1, e1); Expression<Func<string, Func<bool>>> e3 = tbool => () => a < b && 8 > d || c == d; Expression<Func<bool>> e4 = () => b < 4; Expression<Func<RecordName, bool>> e5 = rn => rn.LastName == "ALFKI"; Expression<Func<StringBuilder>> e6 = () => new StringBuilder { Capacity = 20 }; Expression<Func<string, string>> e7 = word => word == "hello" ? "yes" : "no"; foreach (var item in new Expression[] { e1, e2, e3, e4, e5, e6,e7 }) { Console.WriteLine(item.GetCodeDomSource("vb")); }
Visual Basic Output
Namespace Runtime
Public Class LambdaExpression
Private LastName As String
Private Sub New()
MyBase.New
Me.Func`2
End Sub
Private Function Func`2(ByVal rn As Demo.Program.RecordName) As Boolean
Return (LastName Is "ALFKI")
End Function
End Class
End NamespaceC# Output
namespace Runtime { using System; public class LambdaExpression { private string LastName; private LambdaExpression() { this.Func`2; } private bool Func`2(Demo.Program.RecordName rn) { return (LastName == "ALFKI"); } } }
Conclusion
Codedom is too much C# centric, so it’s hard to make it available for every language. The difference between Code Statement and Code Expressions sometimes makes it hard to convert from expression trees.
On the on the other hand Expression trees are too much LINQ oriented. They are less powerful than CodeDom but more easy to express. In expression trees everything is an expression unlike CodeDom. Some constructs are missing from expression trees like the assignment, but we will probably see the improvements in the expression trees in the future. So it might not be a true DSL or language generator, but sure it is enough to get the most of the databases.
There are some other more powerful meta-programming tools and libraries. F# quotation library supports all the available full-set language features expressed as quotations. Dynamic Language Runtime is another expression tree like library focussed more on compiler developers.
Finally this library is not build for runtime code conversion from expression tree to CodeDom, although it is possible. The CodeDom generated code is mainly for debugging to print the source code of the query. It might also be helpful for seeing what is going on under the hood.
) XDocument is an object that take the best features of XMLDocument and XMLReader. It doesn’t load the complete stream, it is possible to read forward and backward. It is very flexible, we can do transformation, parsing, writing, reading, querying etc. 
