Replace parameter into expression

Question

Given:

var paramA = Expression.Parameter(typeof(string), "a");

and:

Expression<Func<string, bool>> expr = b => b == "Something";

Is there anyway to replace b by paramA into the expression expr?

Yes, use `ExpressionVisitor`. There are many examples of parameter replacers on SO, here are mine https://stackoverflow.com/search?q=user%3A5202563+ParameterReplacer — Ivan Stoev, Jun 04 '17 at 15:23
Do you need a replacement, or you simply want an `Expression>` that does the same thing as `expr`, of which `a` is a parameter? — Sergey Kalinichenko, Jun 04 '17 at 15:37
Hi @dasblinkenlight, a replacement or a new expression could work. — Sebastián Guerrero, Jun 04 '17 at 15:45

score 3 · Accepted Answer · answered Jun 04 '17 at 15:45

3

You can walk the expression tree of expr, and replace all occurrences of b with paramA using the approach described in this Q&A: "Combine two lambda expressions with inner expression".

However, if you simply need a lambda expression that uses paramA as its parameter, it is easier to make a lambda that wraps expr instead:

var res = (Expression<Func<string,bool>>)Expression.Lambda(
    Expression.Invoke(expr, paramA)
,   paramA
);

answered Jun 04 '17 at 15:45

Thx @dasblinkenlight ! – Sebastián Guerrero Jun 04 '17 at 16:06
3

I always forget about this approach (mainly because it's not supported in EF which I use exclusively :) +1 – Ivan Stoev Jun 04 '17 at 17:14

1 Answers1