String or Binary Data Would be Truncated…

protected void trigger()
{  
    try
    {
        DataSet ds = ExamManagement.SP.table2_SP_Selectall().GetDataSet();
        if (ds.Tables[0].Rows.Count > 0)
        {
            string a = RandomNumberGenerator(4);
            string b = RandomNumberGenerator(4);
            string c = RandomNumberGenerator(4);
            ExamManagement.SP.table1_insert(a,b,c).Execute();
        }
    }
    catch (SqlException ex)
    {
        ClientMessaging("Error :"+ex);
    }
}

public static string RandomNumberGenerator(int length)
{
    System.Security.Cryptography.RandomNumberGenerator rng = System.Security.Cryptography.RandomNumberGenerator.Create();

    char[] chars = new char[length];

    //based on your requirment you can take only alphabets or number 
    string validChars = "aAbBcCdDeEfFgGhHiIjJkKlLmMnNoOpPqQrRsStTuUvVwWxXzZ";
    byte[] bytes = new byte[length-1];
    for (int i = 0; i < length; i++)
    {

        rng.GetBytes(bytes);

        Random rnd = new Random(bytes[0]);

        chars[i] = validChars[rnd.Next(validChars.Length)];

    }

    return (new string(chars));
}

Column1, varchar(max),null
Column2, varchar(max),null
Column3, varchar(max),null