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c#expression-trees

variable '' of type '' referenced from scope '', but it is not defined

Well, the following code is self-explaining; I want to combine two expressions into one using And operator. The last line causes rune-time the error:

Additional information: variable 'y' of type 'System.String' referenced from scope '', but it is not defined

Code:

Expression<Func<string, bool>> e1 = y => y.Length < 100;
Expression<Func<string, bool>> e2 = y => y.Length < 200;

var e3 = Expression.And(e1.Body, e2.Body);

var e4 = Expression.Lambda<Func<string, bool>>(e3, e1.Parameters.ToArray());
e4.Compile(); // <--- causes run-time error
Solution

  • Thanks everybody collaborated.

    As @dasblinkenlight pointed out the two parameters in the two expressions are not the same. Reason? Well, it is the compiler trick. When compiling, it creates a class for each expression and name each parameter something like xxx1, xxx2,... completely different from the original names.

    And the answer for .Net 4.0+:

    How to Combine two lambdas