How to get distinct instance from a list by Lambda or LINQ

Question

Both Marc‘s and dahlbyk‘s answers seem to work very well. I have a much simpler solution though. Instead of using Distinct, you can use GroupBy. It goes like this:

var listDistinct
    = list.GroupBy(
        i => i.value1,
        (key, group) => group.First()
    ).ToArray();

Notice that I’ve passed two functions to the GroupBy(). The first is a key selector. The second gets only one item from each group. From your question, I assumed First() was the right one. You can write a different one, if you want to. You can try Last() to see what I mean.

I ran a test with the following input:

var list = new [] {
    new { value1 = "ABC", objT = 0 },
    new { value1 = "ABC", objT = 1 },
    new { value1 = "123", objT = 2 },
    new { value1 = "123", objT = 3 },
    new { value1 = "FOO", objT = 4 },
    new { value1 = "BAR", objT = 5 },
    new { value1 = "BAR", objT = 6 },
    new { value1 = "BAR", objT = 7 },
    new { value1 = "UGH", objT = 8 },
};

The result was:

//{ value1 = ABC, objT = 0 }
//{ value1 = 123, objT = 2 }
//{ value1 = FOO, objT = 4 }
//{ value1 = BAR, objT = 5 }
//{ value1 = UGH, objT = 8 }

I haven’t tested it for performance. I believe that this solution is probably a little bit slower than one that uses Distinct. Despite this disadvantage, there are two great advantages: simplicity and flexibility. Usually, it’s better to favor simplicity over optimization, but it really depends on the problem you’re trying to solve.

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