How to check the last character of a string in C#?

I assume you don't actually want the last character position (which would be yourString.Length - 1), but the last character itself. You can find that by indexing the string with the last character position:

yourString[yourString.Length - 1]

Use the endswith method of strings:

if (string.EndsWith("A") || string.EndsWith("B"))
{
    //do stuff here
}

Heres the MSDN article explaining this method:

http://msdn.microsoft.com/en-us/library/system.string.endswith(v=vs.71).aspx

string is a zero based array of char.

char last_char = mystring[mystring.Length - 1];

Regarding the second part of the question, if the char is A, B, C

Using if statement

char last_char = mystring[mystring.Length - 1];
if (last_char == 'A' || last_char == 'B' || last_char == 'C')
{
    //perform action here
}

Using switch statement

switch (last_char)
{
case 'A':
case 'B':
case 'C':
    // perform action here
    break
}