Q:

Casting vs using the 'as' keyword in the CLR

When programming interfaces, I've found I'm doing a lot of casting or object type conversion.
Is there a difference between these two methods of conversion?  If so, is there a cost difference or how does this affect my program?
public interface IMyInterface
{
    void AMethod();
}

public class MyClass : IMyInterface
{
    public void AMethod()
    {
       //Do work
    }

    // Other helper methods....
}

public class Implementation
{
    IMyInterface _MyObj;
    MyClass _myCls1;
    MyClass _myCls2;

    public Implementation()
    {
        _MyObj = new MyClass();

        // What is the difference here:
        _myCls1 = (MyClass)_MyObj;
        _myCls2 = (_MyObj as MyClass);
    }
}

Also, what is "in general" the preferred method?

A:

The answer below the line was written in 2008.
C# 7 introduced pattern matching, which has largely replaced the as operator, as you can now write:
if (randomObject is TargetType tt)
{
    // Use tt here
}

Note that tt is still in scope after this, but not definitely assigned. (It is definitely assigned within the if body.) That's slightly annoying in some cases, so if you really care about introducing the smallest number of variables possible in every scope, you might still want to use is followed by a cast.

I don't think any of the answers so far (at the time of starting this answer!) have really explained where it's worth using which.

Don't do this:
// Bad code - checks type twice for no reason
if (randomObject is TargetType)
{
    TargetType foo = (TargetType) randomObject;
    // Do something with foo
}

Not only is this checking twice, but it may be checking different things, if randomObject is a field rather than a local variable. It's possible for the "if" to pass but then the cast to fail, if another thread changes the value of randomObject between the two.
If randomObject really should be an instance of TargetType, i.e. if it's not, that means there's a bug, then casting is the right solution. That throws an exception immediately, which means that no more work is done under incorrect assumptions, and the exception correctly shows the type of bug.
// This will throw an exception if randomObject is non-null and
// refers to an object of an incompatible type. The cast is
// the best code if that's the behaviour you want.
TargetType convertedRandomObject = (TargetType) randomObject;

If randomObject might be an instance of TargetType and TargetType is a reference type, then use code like this:
TargetType convertedRandomObject = randomObject as TargetType;
if (convertedRandomObject != null)
{
    // Do stuff with convertedRandomObject
}

If randomObject might be an instance of TargetType and TargetType is a value type, then we can't use as with TargetType itself, but we can use a nullable type:
TargetType? convertedRandomObject = randomObject as TargetType?;
if (convertedRandomObject != null)
{
    // Do stuff with convertedRandomObject.Value
}

(Note: currently this is actually slower than is + cast. I think it's more elegant and consistent, but there we go.)
If you really don't need the converted value, but you just need to know whether it is an instance of TargetType, then the is operator is your friend. In this case it doesn't matter whether TargetType is a reference type or a value type.
There may be other cases involving generics where is is useful (because you may not know whether T is a reference type or not, so you can't use as) but they're relatively obscure.
I've almost certainly used is for the value type case before now, not having thought of using a nullable type and as together :)

EDIT: Note that none of the above talks about performance, other than the value type case, where I've noted that unboxing to a nullable value type is actually slower - but consistent.
As per naasking's answer, is-and-cast or is-and-as are both as fast as as-and-null-check with modern JITs, as shown by the code below:
using System;
using System.Diagnostics;
using System.Linq;

class Test
{
    const int Size = 30000000;

    static void Main()
    {
        object[] values = new object[Size];
        for (int i = 0; i < Size - 2; i += 3)
        {
            values[i] = null;
            values[i + 1] = "x";
            values[i + 2] = new object();
        }
        FindLengthWithIsAndCast(values);
        FindLengthWithIsAndAs(values);
        FindLengthWithAsAndNullCheck(values);
    }

    static void FindLengthWithIsAndCast(object[] values)        
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            if (o is string)
            {
                string a = (string) o;
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Is and Cast: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithIsAndAs(object[] values)        
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            if (o is string)
            {
                string a = o as string;
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("Is and As: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }

    static void FindLengthWithAsAndNullCheck(object[] values)        
    {
        Stopwatch sw = Stopwatch.StartNew();
        int len = 0;
        foreach (object o in values)
        {
            string a = o as string;
            if (a != null)
            {
                len += a.Length;
            }
        }
        sw.Stop();
        Console.WriteLine("As and null check: {0} : {1}", len,
                          (long)sw.ElapsedMilliseconds);
    }
}

On my laptop, these all execute in about 60ms. Two things to note:

There's no significant difference between them. (In fact, there are situations in which the as-plus-null-check definitely is slower. The above code actually makes the type check easy because it's for a sealed class; if you're checking for an interface, the balance tips slightly in favour of as-plus-null-check.)
They're all insanely fast. This simply will not be the bottleneck in your code unless you really aren't going to do anything with the values afterwards.

So let's not worry about the performance. Let's worry about correctness and consistency.
I maintain that is-and-cast (or is-and-as) are both unsafe when dealing with variables, as the type of the value it refers to may change due to another thread between the test and the cast. That would be a pretty rare situation - but I'd rather have a convention which I can use consistently.
I also maintain that the as-then-null-check gives a better separation of concerns. We have one statement which attempts a conversion, and then one statement which uses the result. The is-and-cast or is-and-as performs a test and then another attempt to convert the value.
To put it another way, would anyone ever write:
int value;
if (int.TryParse(text, out value))
{
    value = int.Parse(text);
    // Use value
}

That's sort of what is-and-cast is doing - although obviously in a rather cheaper way.

A:

"as" will return NULL if not possible to cast.
casting before will raise an exception.
For the performance, raising an exception is usually more costly in time.

A:

Here's another answer, with some IL comparison. Consider the class:
public class MyClass
{
    public static void Main()
    {
        // Call the 2 methods
    }

    public void DirectCast(Object obj)
    {
        if ( obj is MyClass)
        { 
            MyClass myclass = (MyClass) obj; 
            Console.WriteLine(obj);
        } 
    } 

    public void UsesAs(object obj) 
    { 
        MyClass myclass = obj as MyClass; 
        if (myclass != null) 
        { 
            Console.WriteLine(obj);
        } 
    }
}

Now look at the IL each method produces. Even if the op codes mean nothing to you, you can see one major difference - isinst is being called followed by castclass in the DirectCast method. So two calls instead of one basically.
.method public hidebysig instance void  DirectCast(object obj) cil managed
{
  // Code size       22 (0x16)
  .maxstack  8
  IL_0000:  ldarg.1
  IL_0001:  isinst     MyClass
  IL_0006:  brfalse.s  IL_0015
  IL_0008:  ldarg.1
  IL_0009:  castclass  MyClass
  IL_000e:  pop
  IL_000f:  ldarg.1
  IL_0010:  call       void [mscorlib]System.Console::WriteLine(object)
  IL_0015:  ret
} // end of method MyClass::DirectCast

.method public hidebysig instance void  UsesAs(object obj) cil managed
{
  // Code size       17 (0x11)
  .maxstack  1
  .locals init (class MyClass V_0)
  IL_0000:  ldarg.1
  IL_0001:  isinst     MyClass
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  brfalse.s  IL_0010
  IL_000a:  ldarg.1
  IL_000b:  call       void [mscorlib]System.Console::WriteLine(object)
  IL_0010:  ret
} // end of method MyClass::UsesAs

The isinst keyword versus the castclass
This blog post has a decent comparison between the two ways of doing it. His summary is:

In a direct comparison, isinst is quicker than castclass (although only slightly)
When having to perform checks to ensure the conversion was successful, isinst was significantly quicker than castclass
A combination of isinst and castclass should not be used as this was far slower than the quickest "safe" conversion (over 12% slower)

I personally always use As, because it's easy to read and is recommended by the .NET development team (or Jeffrey Richter anyway)

