Here are another two ways:
string text = "X:Y,A:B,C:D,M:N";
string[] parts = text.Split(',');
string[] identifiers1 = parts
.Select(part => part.Substring(0, 1))
.ToArray();
Console.WriteLine(string.Join(", ", identifiers1));
string[] identifiers2 = new string[parts.Length];
for (int i = 0; i < parts.Length; i++)
{
identifiers2[i] = parts [i].Substring(0,1);
}
Console.WriteLine(string.Join(", ", identifiers2));
Outputs:
X, A, C, M
X, A, C, M