score:2

Accepted answer

The lambda in your example has "closed over" the testValue variable, meaning the compiler has captured it as a field of the same name in an automatically generated class called ConsoleApplication1.Program+<>c__DisplayClass1>. You can use normal reflection to get the current value of that field by casting the right hand-side of the binary expression into a MemberExpression.

var testValue = "hello";
var expr = (Expression<Func<string, bool>>) (x => x == testValue);
var rhs = (MemberExpression) ((BinaryExpression) expr.Body).Right;
var obj = ((ConstantExpression) rhs.Expression).Value;
var field = (FieldInfo) rhs.Member;
var value = field.GetValue(obj);
Debug.Assert(Equals(value, "hello"));
testValue = "changed";
value = field.GetValue(obj);
Debug.Assert(Equals(value, "changed"));

Alternatively you can change your variable into a constant.

const string testValue = "hello";
var expr = (Expression<Func<string, bool>>) (x => x == testValue);
var value = ((ConstantExpression) ((BinaryExpression) expr.Body).Right).Value;
Debug.Assert(Equals(value, "hello"));

score:0

Instead of doing this by yourself, have a look at PartialEvaluator from Matt Warren. It replaces all references to constants with the constants themselves.


Related Articles