How to change a type in an expression tree?

c# expression-trees

Question

I have a method like this:

private bool Method_1(Expression<Func<IPerson, bool>> expression)
{
    /* Some code that will call Method_2 */
}

In this method I want to change the IPerson type to another type. I want to call another method that looks like this:

private bool Method_2(Expression<Func<PersonData, bool>> expression)
{
    /* Some code */
}

So, in method_1 I need to change IPerson to PersonData. How can I do this?

Edit:

When I call: Method_1(p => p.Id == 1) I want to 'save' the condition (p.Id == 1) but I want to execute this condition on another type, namely IPerson. So, I need to alter the expression or create a new expression with IPerson

EDIT II:

For those who are interested, this is (for now) my solution:

private class CustomExpressionVisitor<T> : ExpressionVisitor
{
    ParameterExpression _parameter;

    public CustomExpressionVisitor(ParameterExpression parameter)
    {
        _parameter = parameter;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        return _parameter;
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Member.MemberType == System.Reflection.MemberTypes.Property)
        {
            MemberExpression memberExpression = null;
            var memberName = node.Member.Name;
            var otherMember = typeof(T).GetProperty(memberName);
            memberExpression = Expression.Property(Visit(node.Expression), otherMember);
            return memberExpression;
        }
        else 
        {
            return base.VisitMember(node);
        }
    }
}

And this is the way I use it:

public virtual bool Exists(Expression<Func<Dto, bool>> expression)
{
    var param = Expression.Parameter(typeof(I));
    var result = new CustomExpressionVisitor<I>(param).Visit(expression.Body);
    Expression<Func<I, bool>> lambda = Expression.Lambda<Func<I, bool>>(result, param);

    bool exists = _repository.Exists(lambda);
    return exists;
}

Accepted Answer

It is easy if you use .net 4 (update: as noted in comment ExpressionVisitor was added in version 4 not 4.5) it would require some googling for older frameworks:

There are some assumptions but I think they are valid for your DTO and Entity scenario - properties accessed must match.

class PersonData
{
    public bool Prop { get; set; }
}

interface IPerson 
{
    bool Prop { get; set; }
}

In .net 4 there is ExpressionVisitor class defined that makes this a lot easier if you use older one then you need to write or find implementation of it:

class Visitor<T> : ExpressionVisitor
{
    ParameterExpression _parameter;

    //there must be only one instance of parameter expression for each parameter 
    //there is one so one passed here
    public Visitor(ParameterExpression parameter)
    {
        _parameter = parameter;
    }

    //this method replaces original parameter with given in constructor
    protected override Expression VisitParameter(ParameterExpression node)
    {
        return _parameter;
    }

    //this one is required because PersonData does not implement IPerson and it finds
    //property in PersonData with the same name as the one referenced in expression 
    //and declared on IPerson
    protected override Expression VisitMember(MemberExpression node)
    {
        //only properties are allowed if you use fields then you need to extend
        // this method to handle them
        if (node.Member.MemberType != System.Reflection.MemberTypes.Property)
            throw new NotImplementedException();

        //name of a member referenced in original expression in your 
        //sample Id in mine Prop
        var memberName = node.Member.Name;
        //find property on type T (=PersonData) by name
        var otherMember = typeof(T).GetProperty(memberName);
        //visit left side of this expression p.Id this would be p
        var inner = Visit(node.Expression);
        return Expression.Property(inner, otherMember);
    }
}

Proof of concept:

class Program
{
   static void Main()
    {
        //sample expression
        Expression<Func<IPerson, bool>> expression = x => x.Prop;

        //parameter that will be used in generated expression
        var param = Expression.Parameter(typeof(PersonData));
        //visiting body of original expression that gives us body of the new expression
        var body = new Visitor<PersonData>(param).Visit(expression.Body);
        //generating lambda expression form body and parameter 
        //notice that this is what you need to invoke the Method_2
        Expression<Func<PersonData, bool>> lambda = Expression.Lambda<Func<PersonData, bool>>(body, param);
        //compilation and execution of generated method just to prove that it works
        var boolValue = lambda.Compile()(new PersonData());
    }
}

Note that this will work for simple expressions. If you need to handle x.Prop.Prop1 < 3 then you need to extend this further.



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