Local variable and expression trees

Question

I am learning expression trees in C#.

I am stuck now for a while:

string filterString = \"ruby\";
Expression> expressi         


        

Answer 1

An old question but I came to it when trying to do something similar building expressions for Linq-to-entities (L2E) In that case you cannot use Expression.Block as it cannot be parsed down to SQL.

Here is an explicit example following Jon's answer which would work with L2E. Create a helper class to contain the value of the filter:

class ExpressionScopedVariables
{
    public String Value;
}

Build the tree thus:

var scope = new ExpressionScopedVariables { Value = filterString};
var filterStringExp = Expression.Constant(scope);
var getVariable = typeof(ExpressionScopedVariables).GetMember("Value")[0];
var access = Expression.MakeMemberAccess(filterStringExp, getVariable);

And then replace the constant in the original code with the member access expression:

BinaryExpression equals = Expression.Equal(stringParam, access);
Expression> lambda1 =
    Expression.Lambda>(
        equals,
        new ParameterExpression[] { stringParam });