Errors

"Object Reference not set to an instance of an object"
Object has not been instantiated, for example in VB.NET you probably used the following code:

 Dim sql As SqlClient.SqlCommand

has to be:

 Dim sql As new SqlClient.SqlCommand



“Procedure xxx expects parameter '@ParameterName' which was not supplied”

This can have multiple reasons:
1) a parameter value has been set to null, instead of DBNull.Value. For example:

 mySqlCommand.Parameters["@ParameterName"].Value = null;

has to be:

 mySqlCommand.Parameters["@ParameterName"].Value = DBNull.Value;

2) a parameter value has not been set