How to store the result of a mysql query into a C# string variable?

try like this.... ..
 

Query("SELECT SaksNummer FROM casetracking")
 
    public static string Query(string query)
    {
        string x;
        mysqlCon.Open();
        cmd = new MySqlCommand(query, mysqlCon);
        x = cmd.ExecuteScalar().ToString();
        mysqlCon.Close();
        return x;
    }

First off, don't do it like that! Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
 
Second, if your query worked, then you will load the last of any returned values into your "user" variable (because it is in a loop and the last value will overwrite all the others) - and then you will throw it away without using it because "user" goes out of scope at the end of the loop.
 
At no point do you modify or even reference a variable called currentUsername so it will have the same value at the end as it did at the start...
 
I think you need to sit down and think about what you are tryingto achieve here - because this looks a lot like either very muddled thinking, or the old "chuck it together and hope it works by magic" approach. Neither of these are a good idea...

Try This ->

 static string Put_In_String()
        {
            SqlConnection myConnection = new SqlConnection();
 
                myConnection.Open();
 

            SqlCommand cmd = new SqlCommand();
            cmd.CommandText = "select Query";
			var result = cmd.ExecuteReader();
			var myArray = new string[,] {};
			var counter = 0;
 
			while (result.Read())
			{
				myArray[counter, 0] = (string) result["First Column Name"];
				myArray[counter, 1] = (string) result["Second Column Name"];
				// etc...

				counter++;
			}
            result.Close();
            myConnection.Close();
            return myArray ;
        }