Checking an objects inheritance

[cnurse]

In the following code pTable is of type CustomerTable, which inherits from DataTable. How can I change the "if" statement to check that pTable is derived from DataTable as the statement currently tests false, because the types are different. I thought I would cast ptable, but the of course that would throw an exception if that cast is invalid. if (pTable != typeof(DataTable)) { throw new EwGeneralException(lStk, EwExceptionID.DdlNotDataTable); } Many thanks for reading. Nursey

[jqd2001]

Can you try if (typeof(pTable).BaseType !=typeof(DataTable)) .... James

[Heath Stewart]

You're comparing apples and oranges, there. pTable is an instance, and the typeof operator will always return a Type, so the two would never match unless pTable was a variable which held a Type. If you want to check is something is an instance of a particular type, simply use the is operator:

if (pTable is DataTable)
{
// ...
}

You can also use something like this, although it requires more code and more instructions (when compiled to IL):

if (pTable != null)
{
if (pTable.GetType() == typeof(DataTable))
{
// ...
}
}

There's other ways of doing this, but you don't have to take inheritance into account. If CustomerTable inherits form DataTable, then any instance is a type of CustomerTable, DataTable, MarshalByValueComponent, and - as with everything in .NET - Object. It's also an instance of any interfaces that any classes up the inheritence hierarchy implement. This isn't true going down the hierarchy, though. For example, if you instantiate a new instance of a DataTable, then it is not an instance of a CustomerTable. Also keep in mind that it's not the declaration that counts, its the definition. For example:

object o = new DataTable("Example");

While o is declared as an object, it's still an instance of a DataTable. Calling o.GetType will return System.Data.DataTable. This is polymorphism.

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