Making a custom class IQueryable

Question

I have been working with the TFS API for VS2010 and had to query FieldCollection which I found isn\'t supported by LINQ so I wanted to create a custom class to make the Field an

Answer 1

In your case of wrapping and building a class around an existing IEnumerable Collection type i.e. List<Field>,

you might just use a set of "forward function" wrappers that expose the IQueryable<Field> interface:

public class WorkItemFieldCollection : IEnumerable<Field>, IQueryable<Field>
{
    ...
    #region Implementation of IQueryable<Field>

    public Type ElementType
    {
        get
        {
            return this._fieldList.AsQueryable().ElementType;
        }
    }

    public Expression Expression
    {
        get
        {
            return this._fieldList.AsQueryable().Expression;
        }
    }

    public IQueryProvider Provider
    {
        get
        {
            return this._fieldList.AsQueryable().Provider;
        }
    }

    #endregion
    ...
}

You can now directly query your WorkItemFieldCollection:

var workItemFieldCollection = new WorkItemFieldCollection(...);
var Fuzz = "someStringId";
var workFieldItem = workItemFieldCollection.FirstOrDefault( c => c.Buzz == Fuzz );

Answer 2

If you want an object to be usable by LINQ, implement IEnumerable<T>. IQueryable<T> is overkill for LINQ to Objects. It is designed for converting the expressions into another form.

Or if you want, you can do this

FieldCollection someFieldCollection = ...
IEnumerable<Field> fields = someFieldCollections.Cast<Field>();