Using column name when using SqlDataReader.IsDBNull

You are looking for SqlDataReader.GetOrdinal

According to MSDN

Gets the column ordinal, given the name of the column.

if (read.Read())
{
   int colIndex = read.GetOrdinal("MyColumnName");
   maskedTextBox2.Text = read.IsDBNull(colIndex) ? 
                  string.Empty : 
                  read.GetDateTime(colIndex).ToString("MM/dd/yyyy");

}

As a side note, your query is open to sql injection. Do not use a string concatenation to build a sql command but use a parameterized query

  string query = "SELECT * FROM zajezd WHERE event=@p1 AND year=@p2";
  using(SqlCommand cmd= new SqlCommand(query, spojeni))
  {
     spojeni.Open();
     cmd.Parameters.AddWithValue("@p1",thisrow);
     cmd.Parameters.AddWithValue("@p2",klientClass.Year().ToString());
     using(SqlDataReader read= cmd.ExecuteReader())
     {
       ......
     }
  }

I would try (string)(reader["ColumnName"] == DBNull.Value ? "" : reader["ColumnName"]); to do it by the column name.

You could use GetOrdinal to create your own IsDBNull(string name) extension method.

[DebuggerStepThrough]
public static class SqlDataReaderExtensions
{
    /// <summary>Gets a value that indicates whether the column contains non-existent or missing values.</summary>
    /// <param name="name">The name of the column. </param>
    /// <returns> <see langword="true" /> if the specified column value is equivalent to <see cref="T:System.DBNull" />; otherwise <see langword="false" />.</returns>
    /// <exception cref="T:System.IndexOutOfRangeException">The name specified is not a valid column name. </exception>
    public static bool IsDBNull(this SqlDataReader reader, string name)
    {
        int columnOrdinal = reader.GetOrdinal(name);
        return reader.IsDBNull(columnOrdinal);
    }
}

// Usage
if (read.Read())
{
    maskedTextBox2.Text = read.IsDBNull("MyColumnName") ? 
                  string.Empty : 
                  read.GetDateTime("MyColumnName").ToString("MM/dd/yyyy");

}