How do I call methods with reference variables with Expression Trees

Question

I am trying to figure out how to create an Expression that calls a method which has a reference parameter.

Let me explain my question with a simple (but artificial) example. Consider the method:

    public static int TwiceTheInput(int x)
    {
        return x*2;
    }

I can create an Expression to call the above method by doing something like:

    {
        var inputVar = Expression.Variable(typeof (int), "input");
        var blockExp =
            Expression.Block(
                    new[] {inputVar}
                    , Expression.Assign(inputVar, Expression.Constant(10))
                    , Expression.Assign(inputVar, Expression.Call(GetType().GetMethod("TwiceTheInput", new[] { typeof(int) }), inputVar))
                    , inputVar
                    );
        var result = Expression.Lambda<Func<int>>(blockExp).Compile()();
    }

On execution, the "result" above should end up with a value of 20. Now consider a version of TwiceTheInput() that uses by-reference parameters:

    public static void TwiceTheInputByRef(ref int x)
    {
        x = x * 2;
    }

How do I write a similar Expression Tree to call TwiceTheInputByRef() and pass arguments by reference to it?

Solution: (Thanks to Cicada). Use:

Type.MakeByRefType()

Here's a code segment to generate the Expression Tree:

        {
        var inputVar = Expression.Variable(typeof(int), "input");
        var blockExp =
            Expression.Block(
                    new[] { inputVar }
                    , Expression.Assign(inputVar, Expression.Constant(10))
                    , Expression.Call(GetType().GetMethod("TwiceTheInputByRef", new[] { typeof(int).MakeByRefType() }), inputVar)
                    , inputVar
                    );
        var result = Expression.Lambda<Func<int>>(blockExp).Compile()();
    }

Have you tried using a lambda expression to call the same method, getting the C# compiler to convert that to an expression tree, and then decompiling? That's normally what I do to work out how to build expression trees :) — Jon Skeet
– Jon Skeet, Commented Feb 18, 2013 at 15:55
No, I haven't done it before. Any url that walks through an example? — DPrb
– DPrb, Commented Feb 18, 2013 at 16:02

Alexei Levenkov · Accepted Answer · 2013-02-18 16:26:00Z

9

You don't have to change much, just remove the Assign and change typeof(int) to typeof(int).MakeByRefType().

var blockExp = Expression.Block(
    new[] { inputVar }
    , Expression.Assign(inputVar, Expression.Constant(10))
    , Expression.Call(
       typeof(Program).GetMethod( 
           "TwiceTheInputByRef", new [] { typeof(int).MakeByRefType() }),
       inputVar)
    , inputVar
);

edited Feb 18, 2013 at 16:26

Alexei Levenkov

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answered Feb 18, 2013 at 16:00

user703016

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4 Comments

JerKimball Over a year ago

Don't recall off the top of my head, but could one use the (semi undocumented) __makeref keyword here? You'd need an actual variable, naturally.

user703016 Over a year ago

@JerKimball As you said indeed, __makeref works on variables, not types, so we can't use it here. Note that the MakeByRefType part is just used by GetMethod to resolve the appropriate overload of TwiceTheInputByRef: if there's no overload, that second argument is superfluous.

DPrb Over a year ago

Does Type.MakeByRefType() work equally well for both ref and out parameters? (It seems to work in a quick test that I tried..)

user703016 Over a year ago

Yes, an out parameter is "by ref".

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