I cannot find the way to convert from Expression<Func<T1,bool>> to Expression<Func<T2,bool>>. Since Im using a lot of reflection, in fact, what I really need is a method which takes a type parameter and performs the conversion.
public object Convert(Expression<Func<T1,bool>> expr, Type t);
T2 is derived from T1
public class T1 {
int FamilyId {get; set;}
}
public class T2 : T1 {
... other properties
}
I am defining a filter expression on the base class
Expression<Func<T1,bool>> filter = p => p.FamilyId == [some value]
that i want to apply to a List<T2>
Is this what you're looking for? There are two flavors of the method: the first lets you pass in the new input type as an argument; the second lets you pass in the input type as a generic parameter and get a strongly typed LambdaExpression.
public static LambdaExpression ChangeInputType<T, TResult>(Expression<Func<T, TResult>> expression, Type newInputType)
{
if (!typeof(T).IsAssignableFrom(newInputType))
throw new Exception(string.Format("{0} is not assignable from {1}.", typeof(T), newInputType));
var beforeParameter = expression.Parameters.Single();
var afterParameter = Expression.Parameter(newInputType, beforeParameter.Name);
var visitor = new SubstitutionExpressionVisitor(beforeParameter, afterParameter);
return Expression.Lambda(visitor.Visit(expression.Body), afterParameter);
}
public static Expression<Func<T2, TResult>> ChangeInputType<T1, T2, TResult>(Expression<Func<T1, TResult>> expression)
{
if (!typeof(T1).IsAssignableFrom(typeof(T2)))
throw new Exception(string.Format("{0} is not assignable from {1}.", typeof(T1), typeof(T2)));
var beforeParameter = expression.Parameters.Single();
var afterParameter = Expression.Parameter(typeof(T2), beforeParameter.Name);
var visitor = new SubstitutionExpressionVisitor(beforeParameter, afterParameter);
return Expression.Lambda<Func<T2, TResult>>(visitor.Visit(expression.Body), afterParameter);
}
public class SubstitutionExpressionVisitor : ExpressionVisitor
{
private Expression before, after;
public SubstitutionExpressionVisitor(Expression before, Expression after)
{
this.before = before;
this.after = after;
}
public override Expression Visit(Expression node)
{
return node == before ? after : base.Visit(node);
}
}
What your asking for is very unwise. How would the compiler ever know if T1 is can be converted to T2? Seems like asking for awful run-time errors even if it is possible.*
(*I don't think its possible since you are trying to combine refection with nested generic types.)
It looks like you want to combine 2 expressions - T2 to T1 conversion and than call to expr with given result.
This question discusses Combining two expressions (Expression<Func<T, bool>>) in general. For your case I think you need Expression.Call to construct conversion expression and than again to call original expression with result of conversion.
