"The binary operator Add is not defined for the types 'System.String' and 'System.String'." -- Really?

c# expression-trees

Question

When trying to run the following code:

    Expression<Func<string, string>> stringExpression = Expression.Lambda<Func<string, string>>(
        Expression.Add(
            stringParam,
            Expression.Constant("A")
        ),
        new List<ParameterExpression>() { stringParam }
    );

    string AB = stringExpression.Compile()("B");

I get the error referenced in the title: "The binary operator Add is not defined for the types 'System.String' and 'System.String'." Is that really the case? Obviously in C# it works. Is doing string s = "A" + "B" in C# special syntactic sugar that the expression compiler doesn't have access to?

Accepted Answer

It's absolutely right, yes. There is no such operator - the C# compiler converts string + string into a call to string.Concat. (This is important, because it means that x + y + z can be converted into string.Concat(x, y, z) which avoids creating intermediate strings pointlessly.

Have a look at the docs for string operators - only == and != are defined by the framework.


Popular Answer

This just caught me out too, and as Jon points out in his answer, the C# compiler converts string + string into string.Concat. There is an overload of the Expression.Add method that allows you to specify the "add" method to use.

var concatMethod = typeof(string).GetMethod("Concat", new[] { typeof(string), typeof(string) }); 
var addExpr = Expression.Add(Expression.Constant("hello "),Expression.Constant("world"), concatMethod);

You might want to change the string.Concat method to use the correct overload.

Proving this works:

Console.WriteLine(Expression.Lambda<Func<string>>(addExpr).Compile()());

Will output:

hello world



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