How To Make Global Variable In C#

First, create a class called global in your application with the code given below.

class Global
    {
        public static string UserID;

    }

Now, you can access it anywhere in your forms after assigning a value to it.

Global.UserID = comboBox1.Text.ToString();

You can use it in any form, as shown below.

if(Global.UserID == "abdulateef")
            {
                dataGridView1.Refresh();
            string ConnectionString = "DRIVER={MySQL ODBC 3.51 Driver};Server=localhost;Database="";User Id="";Password=";
            string query = "Select filename, encfile, password,date,username FROM encinfor";
            OdbcConnection conn = new OdbcConnection(ConnectionString);
            conn.Open();
            try
            {
                //populating the gridview1 on form load
                DataSet ds = new DataSet();
                using(OdbcDataAdapter adapater = new OdbcDataAdapter(query, conn))
                {
                    adapater.Fill(ds);
                    dataGridView1.DataSource = ds.Tables[0];
                }
            }
            catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

The best practice is to make sure that you don't assign different value to the same global variable. You can have many global variables, as per your desire. For an example, check the code given below.

class Global
    {
        public static string UserID;
        public static string Status;
        public static string Adim;

    }

Thanks, I hope it helps.