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c#linqdappernullreferenceexception

Null Dapper.net query still returning Null Reference Exception with FirstOrDefault()

I would like to return the max Id from a table using Dapper.net

var x = connection.Query<int>("SELECT max(val) FROM info").FirstOrDefault();

This works - unless no row exists then I get an

Object reference not set to an instance of an object.

Shouldn't the 'OrDefault' mean this returns 0 when no records are present?

How can I return 0 - or some non null value to prevent a crash.

thx

Solution

  • The issue is that you're telling Dapper to expect a sequence of int, but you actually have the possiblity of a null value. So you either need to change the type

    var x = connection.Query<int?>("SELECT max(val) FROM info").Single() ?? 0;

    Or you need to change the query to handle the null.

    var x = connection.Query<int>("SELECT COALESCE(max(val), 0) FROM info").Single();

    I'm using Single here because this query should only ever return exactly one row.

    You would use FirstOrDefault when you expect a sequence and only want the first item, or if there are no items you want the default value of the item type.