C# cannot generate a random string [duplicate]

This question already has an answer here:

• Random.Next returns always the same values [duplicate] 4 answers
• Random number generator only generating one random number 9 answers

I cannot work out why C# is doing this.

Here's my code;

private string RandomString(int length)
    {
        const string chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        string randomString = "";

        for(int i = 0; i < length; i++)
        {
            randomString += chars.ToCharArray()[new Random().Next(chars.ToCharArray().Length)];
        }

        return randomString;
    }

First result: "wwwwwwwwwwwwwwwwwwww"

Second result: "ssssssssssssssssssss"

Third result: "mmmmmmmmmmmmmmmmmmmm"