Question

I want to define a Lambda Expression with an out parameter. Is it possible to do it?

Below are code snippets from a C# .Net 4.0 console app that I tried.

As you can see in Procedure25 I can use lambda expressions to define a delegate that has an output parameter, however, when I want to use linq expressions to do the same, the code in procedure 24 fails with:

System.ArgumentException was unhandled Message=ParameterExpression
of type ‘System.Boolean’ cannot be used for delegate parameter of type
‘System.Boolean&’ Source=System.Core

I know I could use an input class object with a bool member and pass back the value to the caller that way but I was curious if I could somehow define out parameters.

Thanks

static void Main(string[] args)
{
  Procedure25();
  Procedure24();
  Console.WriteLine("Done!");
  Console.ReadKey();
}

private delegate int Evaluate(string value, out bool usesVars);

private static void Procedure24()
{

  // This fails to compile:
  //Expression<Evaluate> x = (string val,  out bool usesSimVals) =>
  //{
  //  usesSimVals = true;
  //  Console.WriteLine(val);
  //  return 1;
  //};


  ParameterExpression valueParameter = Expression.Parameter(typeof (string));
  MethodCallExpression methodCall = Expression.Call(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(object) }), valueParameter);

  bool usesVars;
  ParameterExpression usesVarsParameter = Expression.Parameter(typeof (bool), "out usesVars");


  Expression.Lambda<Evaluate>(methodCall, valueParameter, usesVarsParameter).Compile()("test", out usesVars);
  Console.WriteLine(usesVars);

}

private static void Procedure25()
{
  Evaluate x = (string value, out bool vars) => { vars = true;
    Console.WriteLine(value);
                                                    return 1;
  };

  bool usesVars;
  x("test", out usesVars);
}

EDIT:

Ani, awesome, thanks. So the key thing was to call MakeByRefType on the parameter type.

For the record here is a code snippet that works based on Ani’s suggestion:

private static void Procedure24()
{
  ParameterExpression valueParameter = Expression.Parameter(typeof (string));
  MethodCallExpression methodCall = Expression.Call(typeof(Console).GetMethod("WriteLine", new Type[] { typeof(object) }), valueParameter);

  bool usesVars;
  ParameterExpression usesVarsParameter = Expression.Parameter(typeof (bool).MakeByRefType(), "out usesVars");

  Expression block = Expression.Block(methodCall, Expression.Assign(usesVarsParameter, Expression.Constant(true)), Expression.Constant(1));
  int result = Expression.Lambda<Evaluate>(block, valueParameter, usesVarsParameter).Compile()("test", out usesVars);
  Console.WriteLine("Result={0}, usesVars={1}", result, usesVars);

}

Answer 1

You need Type.MakeByRefType:

var usesVarsParameter = Expression.Parameter(typeof(bool).MakeByRefType(), "usesVars");

Note that your code sample has an additional problem: your expression-body isn’t correct – it’s not returning a value when it should be returning an int to satisfy the delegate-type’s return-type.

Here’s a way you can fix that (like your lambda example):

var body = Expression.Block(methodCall, Expression.Constant(1));

Expression.Lambda<Evaluate>(body, valueParameter, usesVarsParameter)
          .Compile()("test", out usesVars);

Also note that you are not assigning the out parameter inside the expression. Expression.Lambda is letting you get away with it, which I didn’t expect, but hey, the BCL doesn’t have to follow the same rules as C#!