CODEWITHSUNDEEP
c#genericscasting
Sjoerd222888
Sjoerd222888

Reputation: 3476

Cast from with generics from long to int using object instance

I am casting dynamically with generics and have come across a problem when casting from long to int. The problem basically boils down to the following:

While this works:

 long l = 10;
 int i = (int) l;

This does not:

 long l = 10;
 object o = l;
 int i = (int)o;

So the problem is that I have an object as variable of type object but the instance behind is of type long and I want to cast this to an int. What I have found so far is this article: Representation and identity (Eric Lippert).

So what would be valid is this:

 long l = 10;
 object o = l;
 int i = (int)(long)o;

What I have tried is this:

 long l = 10;
 object o = l;
 int i = (int) Convert.ChangeType(o, typeof(long));

But this does not work. Now the question is how can I cast dynamically without System.InvalidCastException?

Currently I have is this (and it does NOT work):

public T Parse<T>(object value){
    return (T) Convert.ChangeType(value, value.GetType());
}

How can I make it work and be able to pass an object of type long with T beeing int.

Upvotes: 4

Views: 972

Answers (3)

Sean
Sean

Reputation: 62492

Instead of this:

long l = 10;
object o = l;
int i = (int) Convert.ChangeType(o, typeof(long));

Just ask Convert to give you an int, as that's what you want:

long l = 10;
object o = l;
int i = (int) Convert.ChangeType(o, typeof(int));

The Convert class with use the IConvertable interface to do the conversion for you.

Using this approach you can write your Parse function like this:

T Parse<T>(object value)
{
  return (T) Convert.ChangeType(value, typeof(T));
}

And call it like this:

long l = 10;
object o = l;
int i=Parse<int>(o);

Upvotes: 2

Marc
Marc

Reputation: 993

How about this

return (T) TypeDescriptor.GetConverter(typeof(T)).ConvertFromString(value.ToString());

It will still fail if your long value is too big for an int so you would still need to deal with that.

Look at this Generic TryParse

Upvotes: 0

Parimal Raj
Parimal Raj

Reputation: 20585

long to int is type casting, when you doing this you are just changing value of one type to another similar type, in some case that may lead to value loss, and compiler knows how to perform so.

object to int is type conversion. you are attempting to change the underlying type. Hence the runtime error.

Have a look at Casting and Type Conversion & Boxing & Unboxing

Upvotes: 0

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