Index and count must refer to a location within the string. Parameter name: count.

Without seeing an example of what tbxUserId.Text actually looks like, this can only be a guess but the error is telling you that the count you are using (7) would require going past the end of the string.

The second parameter in Remove() is a count of characters to delete, NOT the position of the last character to delete, in case you are confused.

You need to code more defensively than that, you are assuming that (2, 7) is always going to refer to a position within your string.

But what if I entered '10ABC' into your tbxUserID? Then the second argument (7) isn't valid and you are going to see the error you're struggling with

I don't know what you're trying to do, are you trying to find a string and remove sections that match?

string UserID = tbxUserID.Text;
string UserIDSort = UserID.Remove(2, UserID.Length - 2);

This way, the length of UserID can change and your calculation is still valid

You could probably just do this with a substring if the code is always 2 long?

UserIdSort = UserID.Substring(0, 2);

The rule of thumb is: if you use string.Remove repeatedly on the same string variable or even repeatedly concatenated strings using "+", you should use StringBuilder instead.

Strings are immutable. You never modify a string object using string operation. Instead, you create a brand new string object which is possibly assigned to the same string variable. If you do this over and over, performance suffer. Use StringBuilder for string modification, use StringBuilder.ToString to get back to string type.

—SA