根据MSDN.
http://msdn.microsoft.com/en -us/library/system.data.odbc.odbccommand.parameters.aspx

According to MSDN.
http://msdn.microsoft.com/en-us/library/system.data.odbc.odbccommand.parameters.aspx

当CommandType设置为Text时，用于ODBC的.NET Framework数据提供程序不支持将命名参数传递给SQL语句或OdbcCommand调用的存储过程.在这两种情况下，均使用问号(?)占位符.

When CommandType is set to Text, the .NET Framework Data Provider for ODBC does not support passing named parameters to an SQL statement or to a stored procedure called by an OdbcCommand. In either of these cases, use the question mark (?) placeholder.

所以您的查询应该是:

string sql = "insert into klant (firstname) values (?)"

string sql = "insert into klant (firstname) values (?)"

如果您有多个参数，则会按照添加它们的顺序进行设置.

If you have multiple parameters, they are set in the order you add them.

此外，我认为这行

string firstname = txtfirstname.ToString();

应阅读

string firstname = txtfirstname.Text();

但这不是引起您直接问题的原因.

But that is not what is causing your immediate problem.