MemberExpression: InvalidOperationExpression variable 'x' referenced from scope '', but it is not defined

c# expression-trees linq

Question

I'm using System.Linq.Expressions

I was attempting to build a simple LambdaExpression that includes a MemberExpression. If I create the MemberExpression explicitly with the System.Linq.Expressions API (e.g. MakeMemberAccess), I will get the error "InvalidOperationExpression variable 'x' referenced from scope '', but it is not defined" when I call Compile() on the LambdaExpression.

For example ,this is my code

Expression<Func<Customer, string>> expression1, expression2, expression3;
Func<Customer, string> fn;
expression1 = (x) => x.Title;
fn = expression1.Compile();//works
fn(c);
MemberExpression m;
m = Expression.MakeMemberAccess(
Expression.Parameter(typeof(Customer), "x"), typeof(Customer).GetProperty("Title"));
expression2 = Expression.Lambda<Func<Customer, string>>(m,
    Expression.Parameter(typeof(Customer), "x"));

m = Expression.Property(Expression.Parameter(typeof(Customer),"x"), "Title");
expression3 = Expression.Lambda<Func<Customer, string>>(m,
    Expression.Parameter(typeof(Customer), "x"));

fn = expression3.Compile();//InvalidOperationExpression variable 'x' referenced from scope '', but it is not defined
fn = expression2.Compile();//InvalidOperationExpression variable 'x' referenced from scope '', but it is not defined

expression2 and expression3 throw an exception when the Compile() method is called, but expression1 does not; expression1 works. Why is this? How do I create an MemberExpression like in expressions 2, 3 and get them to work (not throw an exception) when I call Compile()?

Thanks

Accepted Answer

You're creating different parameters called "x" several times. If you use a single ParameterExpression, it should all work fine.

ParameterExpression p = Expression.Parameter(typeof(Customer), "x");
MemberExpression m = Expression.MakeMemberAccess(p, 
    typeof(Customer).GetProperty("Title"));
expression2 = Expression.Lambda<Func<Customer, string>>(m, p);

m = Expression.Property(p, "Title");
expression3 = Expression.Lambda<Func<Customer, string>>(m, p);

fn = expression3.Compile();
fn = expression2.Compile();

Basically parameter expressions aren't matched by name - you've got to use the same one everywhere. It's a bit of a pain, but there we go...




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