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?

Answer 1

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.