Use of C# var for implicit typing of System.Data.D

问题:

foreach (var row in table.Rows)
{
     DoSomethingWith(row);
}

Assuming that I'm working with a standard System.Data.DataTable (which has a collection of System.Data.DataRow objects), the variable 'row' above resolves as an object type, not a System.Data.DataRow.

foreach (DataRow row in table.Rows)
{
     DoSomethingWith(row);
}

Works as I would expect. Is there a particular reason for this?

Thanks.

回答1:

That's because Rows is DataRowCollection, which in turn is IEnumerable and not IEnumerable<DataRow>, which means that type inferred will be object.

When you explicitly state type in foreach, you instruct c# to add cast to each call, which is why it works.

回答2:

An implicit cast happens. Also note that an InvalidCastException can be thrown if the cast isn't possible.

回答3:

table.Rows is a DataRowCollection which is IEnumberable ( and not IEnumerable<T>, T being DataRow), so it is not strongly typed to a DataRow, but a object i.e it is a collection of objects.

There is a DataTable extensions which you can use though - http://msdn.microsoft.com/en-us/library/system.data.datatableextensions.asenumerable.aspx

foreach (var row in table.AsEnumerable())
{

}

回答4:

Try this:

System.Data.DataTable dt = new System.Data.DataTable();

foreach (var row in dt.Rows.Cast<System.Data.DataRow>())
{

}

To use Rows.Cast you have to use System.Linq.