score:2

Accepted answer

You need to use reflection for this. This works:

public class A
{
    public string Name { get; set; }
    public string Color1 { get; set; }
    public string Color2 { get; set; }
    public string Length { get; set; }

    public static IEnumerable<A> Intersecting(IEnumerable<A> input, List<string> propertyNames)
    { 
        if(input == null)
            throw new ArgumentNullException("input must not be null ", "input");
        if (!input.Any() || propertyNames.Count <= 1)
            return input;

        var properties = typeof(A).GetProperties();
        var validNames = properties.Select(p => p.Name);
        if (propertyNames.Except(validNames, StringComparer.InvariantCultureIgnoreCase).Any())
            throw new ArgumentException("All properties must be one of these: " + string.Join(",", validNames), "propertyNames");

        var props = from prop in properties
                    join name in validNames.Intersect(propertyNames, StringComparer.InvariantCultureIgnoreCase)
                    on prop.Name equals name
                    select prop;
        var allIntersecting = input
            .Select(a => new { 
                Object = a,
                FirstVal = props.First().GetValue(a, null),
                Rest = props.Skip(1).Select(p => p.GetValue(a, null)),
            })
            .Select(x => new { 
                x.Object, x.FirstVal, x.Rest,
                UniqueValues = new HashSet<object>{ x.FirstVal }
            })
            .Where(x => x.Rest.All(v => !x.UniqueValues.Add(v)))
            .Select(x => x.Object);
        return allIntersecting;
    }
}

Sample data:

var aList = new List<A> { 
    new A { Color1 = "Red", Length = "2", Name = "Red" }, new A { Color1 = "Blue", Length = "2", Name = "Blue" },
    new A { Color1 = "Red", Length = "2", Name = "A3" }, new A { Color1 = "Blue", Length = "2", Name = "A3" },
    new A { Color1 = "Red", Length = "3", Name = "Red" }, new A { Color1 = "Blue", Length = "2", Name = "A6" },
};
var intersecting = A.Intersecting(aList, new List<string> { "Color1", "Name" }).ToList();

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