Question

I am trying to use a linq expression to filter a list in a generic method and can’t quite get it right.

Here is what it looks like:

the call:

         Expression<Func<SomeObject, object>> screen = i => i.BooleanFlag == false;
         SomeObjectList.Build<SomeObject>(
            screen,
            NonBuiltListOfTypeSomeObject);
        }

the method:

    public void Build<T>(
        Expression<Func<T,object>> screen,
        List<T> items) where T : class
    {
        this.Values = items.Select(f => new ModelForBuild()
        {
            //Build Parameters
        }).ToList();
    }

What I am trying to do is screen the items before they are built into this.Values. I pass in the linq expression trying to deny access based on the boolean property in SomeObject. However, the compiler tells me I cannot use

this.Values = items.Where(screen).Select(f => new ModelForBuild()...

I have tried various ways so maybe there is a better way to do this, if so, what is that way OR how can I effectively include the linq expression to screen the list of items?

EDIT

Error message from .Where(screen)

Error 1 Instance argument: cannot convert from 'System.Collections.Generic.List<T>' to 'System.Linq.IQueryable<T>'

Error 2 'System.Collections.Generic.List<T>' does not contain a definition for 'Where' and the best extension method overload 'System.Linq.Queryable.Where<TSource>(System.Linq.IQueryable<TSource>, System.Linq.Expressions.Expression<System.Func<TSource,bool>>)' has some invalid arguments

Error 24 Argument 2: cannot convert from 'System.Linq.Expressions.Expression<System.Func<T,object>>' to 'System.Linq.Expressions.Expression<System.Func<T,bool>>'


Answer 1

I think you mean something like this:

void Main()
{
    var builder = new Builder();
    var unbuilt = new List<ModelForBuild>();
    builder.Build<ModelForBuild>( m => m.Id == 0, unbuilt);
}

public class Builder
{
    public IEnumerable<ModelForBuild> Values{ get; private set; }

    public void Build<T>( Func<T, bool> screen, 
                          IEnumerable<T> items ) where T : class
    {
        Values = items.Where(screen).Select(
            i => new ModelForBuild{ Id = 1 }).ToList();
    }
}

public class ModelForBuild
{
    public int Id {get;set;}
}

You don’t need to use Expression<T>.