Solution

You can allow users to select a null or a blank value from a drop-down list by adding an empty List Item to the DropDownList. The List Item can have an empty string as its value, and you can set the Text property to represent the blank or null option.

There are certain things that you need to consider:

Thus, you need to handle with the scenario:

For example, if you keep @ContactID as a null/empty string, you need to pass the value as DBNull.Value.

When a @ContactID has value, you must cast it into an integer.

More importantly, you need to provide the parameter with value and type (length) instead of just providing the value via AddWithValue(). You may read the article: >AddWithValue is Evil

Also, make sure that your stored procedure is modified with the ContractID parameter to allow null. As it is nullable and optional, the parameter needs to be placed at the end (mandatory/not nullable parameter(s) comes first).

Answered by: >Yong Shun

Credit: >StackOverflow

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