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  1. PO
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    <p>There are some differences between a CLR null and a DBNull. First, null in relational databases has different "equals" semantics: null is not equal to null. CLR null IS equal to null. </p> <p>But I suspect the main reason is to do with the way parameter default values work in SQL server and the implementation of the provider.</p> <p>To see the difference, create a procedure with a parameter that has a default value:</p> <pre><code>CREATE PROC [Echo] @s varchar(MAX) = 'hello' AS SELECT @s [Echo] </code></pre> <p>Well-structured DAL code should separate command creation from use (to enable using the same command many times, for example to invoke a stored procedure many times efficiently). Write a method that returns a SqlCommand representing the above procedure:</p> <pre><code>SqlCommand GetEchoProc() { var cmd = new SqlCommand("Echo"); cmd.Parameters.Add("@s", SqlDbType.VarChar); return cmd; } </code></pre> <p>If you now invoke the command without setting the @s parameter, or set its value to (CLR) null, it will use the default value 'hello'. If on the other hand you set the parameter value to DBNull.Value, it will use that and echo DbNull.Value.</p> <p>Since there's two different results using CLR null or database null as parameter value, you can't represent both cases with only one of them. If CLR null was to be the only one, it'd have to work the way DBNull.Value does today. One way to indicate to the provider "I want to use the default value" could then be to not declare the parameter at all (a parameter with a default value of course makes sense to describe as an "optional parameter"), but in a scenario where the command object is cached and reused this does lead to removing and re-adding the parameter.</p> <p>I'm not sure if I think DBNull was a good idea or not, but a lot of people are unaware of the things I've mentioned here, so I figured it worth mentioning.</p>
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    1. POWhat is the point of DBNull?
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    1. USThe Dag
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    1. COI've now learned something as well: DBNull does not actually have the equals semantics of database nulls. The second point still stands though - DBNull or null really do work differently as parameter values.
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    2. COI think it is common belief that the DBNull and null discrepancy originally existed because .NET and SQL Server were originally built by separate teams. It's arguable if the two nulls exist now as a legacy of this separate of teams, or as something that actually benefits more than annoys.
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