How to use LINQ Distinct() with multiple fields

Question

I have the following EF class derived from a database (simplified)

class Product
{ 
     public string ProductId;
     public string Product         


        

Answer 1

Answering the headline of the question (what attracted people here) and ignoring that the example used anonymous types....

This solution will also work for non-anonymous types. It should not be needed for anonymous types.

Helper class:

/// 

/// Allow IEqualityComparer to be configured within a lambda expression.
/// From https://stackoverflow.com/questions/98033/wrap-a-delegate-in-an-iequalitycomparer
/// 
/// 
public class LambdaEqualityComparer : IEqualityComparer
{
    readonly Func _comparer;
    readonly Func _hash;

    /// 

    /// Simplest constructor, provide a conversion to string for type T to use as a comparison key (GetHashCode() and Equals().
    /// https://stackoverflow.com/questions/98033/wrap-a-delegate-in-an-iequalitycomparer, user "orip"
    /// 
    /// 
    public LambdaEqualityComparer(Func toString)
        : this((t1, t2) => toString(t1) == toString(t2), t => toString(t).GetHashCode())
    {
    }

    /// 

    /// Constructor.  Assumes T.GetHashCode() is accurate.
    /// 
    /// 
    public LambdaEqualityComparer(Func comparer)
        : this(comparer, t => t.GetHashCode())
    {
    }

    /// 

    /// Constructor, provide a equality comparer and a hash.
    /// 
    /// 
    /// 
    public LambdaEqualityComparer(Func comparer, Func hash)
    {
        _comparer = comparer;
        _hash = hash;
    }

    public bool Equals(T x, T y)
    {
        return _comparer(x, y);
    }

    public int GetHashCode(T obj)
    {
        return _hash(obj);
    }    
}

Simplest usage:

List products = duplicatedProducts.Distinct(
    new LambdaEqualityComparer(p =>
        String.Format("{0}{1}{2}{3}",
            p.ProductId,
            p.ProductName,
            p.CategoryId,
            p.CategoryName))
        ).ToList();

The simplest (but not that efficient) usage is to map to a string representation so that custom hashing is avoided. Equal strings already have equal hash codes.

Reference:
Wrap a delegate in an IEqualityComparer