string startsWith = "somestring:";
// Note: this works with any data type
switch (startsWith)
{
// Using the 'when' keyword you can convert your case to a bool like
// expression like so:
case string when startsWith.StartsWith("somestring:"):
Console.WriteLine("hit");
break;
case string when startsWith.StartsWith("someotherstring:"):
Console.WriteLine("hit 1");
break;
}
// Output: hit