Specified cast not valid.

Question

I am using .NET Framework 4.0 and recently came across a strange problem.

Cast to decimal (decimal)row["ID"] fails with "Specified cast not valid" where row["ID"] is of datatype Int64 and a value of 38990 :

However if i use Convert.ToDecimal(row["ID"]) it works.

Why is the behaviour not consistent - am i missing something here ?

Regards,
Prem

Vulpes · Answer

Casts and the Convert methods work in slightly different ways. As you know row[ID] returns an object reference. In this particular example the object is actually a boxed Int64 (i.e. long in C#) with a value of 38990. In order to unbox this object you need to cast it to its exact type i.e. long though there's nothing to stop you then casting that to another compatible type such as decimal. So I'd expect this line to work: decimal d = (decimal)(long)row[ID]; In the case of Convert.ToDecimal(object) all that's necessary is that whatever the object actually is can be represented as a decimal. The underlying code actually calls Int64's IConvertible.ToDecimal() method and in this case there's no problem in converting the long value directly to a decimal.

Premkumar Ekkaladevi · Answer

Thanks - that was a good explanation.