How do I extract text that lies between parentheses (round brackets)?

A very simple way to do it is by using regular expressions:

Regex.Match("User name (sales)", @"\(([^)]*)\)").Groups[1].Value

As a response to the (very funny) comment, here's the same Regex with some explanation:

\(             # Escaped parenthesis, means "starts with a '(' character"
    (          # Parentheses in a regex mean "put (capture) the stuff 
               #     in between into the Groups array" 
       [^)]    # Any character that is not a ')' character
       *       # Zero or more occurrences of the aforementioned "non ')' char"
    )          # Close the capturing group
\)             # "Ends with a ')' character"

Assuming that you only have one pair of parenthesis.

string s = "User name (sales)";
int start = s.IndexOf("(") + 1;
int end = s.IndexOf(")", start);
string result = s.Substring(start, end - start);

Use this function:

public string GetSubstringByString(string a, string b, string c)
    {
        return c.Substring((c.IndexOf(a) + a.Length), (c.IndexOf(b) - c.IndexOf(a) - a.Length));
    }

and here is the usage:

GetSubstringByString("(", ")", "User name (sales)")

and the output would be:

sales

Regular expressions might be the best tool here. If you are not famililar with them, I recommend you install Expresso - a great little regex tool.

Something like:

Regex regex = new Regex("\\((?<TextInsideBrackets>\\w+)\\)");
string incomingValue = "Username (sales)";
string insideBrackets = null;
Match match = regex.Match(incomingValue);
if(match.Success)
{
    insideBrackets = match.Groups["TextInsideBrackets"].Value;
}

string input = "User name (sales)";

string output = input.Substring(input.IndexOf('(') + 1, input.IndexOf(')') - input.IndexOf('(') - 1);

A regex maybe? I think this would work...

\(([a-z]+?)\)

Use a Regular Expression:

string test = "(test)"; 
string word = Regex.Match(test, @"\((\w+)\)").Groups[1].Value;
Console.WriteLine(word);

input.Remove(input.IndexOf(')')).Substring(input.IndexOf('(') + 1);

The regex method is superior I think, but if you wanted to use the humble substring

string input= "my name is (Jayne C)";
int start = input.IndexOf("(");
int stop = input.IndexOf(")");
string output = input.Substring(start+1, stop - start - 1);

or

string input = "my name is (Jayne C)";
string output  = input.Substring(input.IndexOf("(") +1, input.IndexOf(")")- input.IndexOf("(")- 1);

I've been using and abusing C#9 recently and I can't help throwing in Spans even in questionable scenarios... Just for the fun of it, here's a variation on the answers above:

    var input = "User name (sales)";
    var txtSpan = input.AsSpan();
    var startPoint = txtSpan.IndexOf('(') + 1;
    var length = txtSpan.LastIndexOf(')') - startPoint;
    var output = txtSpan.Slice(startPoint, length);

For the OP's specific scenario, it produces the right output. (Personally, I'd use RegEx, as posted by others. It's easier to get around the more tricky scenarios where the solution above falls apart).

A better version (as extension method) I made for my own project:

//Note: This only captures the first occurrence, but 
//can be easily modified to scan across the text (I'd prefer Slicing a Span)  
public static string ExtractFromBetweenChars(this string txt, char openChar, char closeChar)
{
    ReadOnlySpan<char> span = txt.AsSpan();
    int firstCharPos = span.IndexOf(openChar);
    int lastCharPos = -1;

    if (firstCharPos != -1) 
    { 
        for (int n = firstCharPos + 1; n < span.Length; n++)
        {
            if (span[n] == openChar) firstCharPos = n; //This allows the opening char position to change
            if (span[n] == closeChar) lastCharPos = n;
            if (lastCharPos > firstCharPos) break;
            //This would correctly extract "sales" from this [contrived]
            //example: "just (a (name (sales) )))(test"
        }
        return span.Slice(firstCharPos + 1, lastCharPos - firstCharPos - 1).ToString();
    }
    return "";
}

 var input = "12(34)1(12)(14)234";
 var output = "";
 for (int i = 0; i < input.Length; i++)
 {
     if (input[i] == '(')
     {
         var start = i + 1;
         var end = input.IndexOf(')', i + 1);
         output += input.Substring(start, end - start) + ",";
     }
 }

 if (output.Length > 0) // remove last comma
  output = output.Remove(output.Length - 1);

output : "34,12,14"

Much similar to @Gustavo Baiocchi Costa but offset is being calculated with another intermediate Substring.

int innerTextStart = input.IndexOf("(") + 1;
int innerTextLength = input.Substring(start).IndexOf(")");
string output = input.Substring(innerTextStart, innerTextLength);