.NET Framework 1.1 Performance Guidelines - Boxing and Unboxing

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- J.D. Meier, Srinath Vasireddy, Ashish Babbar, Rico Mariani, and Alex Mackman

Avoid Frequent Boxing and Unboxing Overhead

Boxing causes a heap allocation and a memory copy operation. To avoid boxing, do not treat value types as reference types. Avoid passing value types in method parameters that expect a reference type. Where boxing is unavoidable, to reduce the boxing overhead, box your variable once and keep an object reference to the boxed copy as long as needed, and then unbox it when you need a value type again.

int p = 123;
object box;
box = (object)p;  // Explicit boxing with a cast
//use the box variable instead of p

Note Boxing in Visual Basic .NET tends to occur more frequently than in C# due to the language's pass-by-value semantics and extra calls to GetObjectValue.

Collections and Boxing

Collections store only data with base type as Object. Passing value types such as integers and floating point numbers to collections causes boxing. A common scenario is populating collections with data containing int or float types returned from a database. The overhead can be excessive in the case of collections due to iteration. The problem is illustrated by the following code snippet.

ArrayList al = new ArrayList();
for (int i=0; i<1000;i++)
  al.Add(i); //Implicitly boxed because Add() takes an object
int f = (int)al[0]; // The element is unboxed

To prevent this, consider using an array instead, or creating a custom collection class for your specific value type. You must perform unboxing with an explicit cast operator.

Note The .NET Framework 2.0, at the time of this writing, introduces generics to the C# language. This will make it possible to write variations of the above code with no boxing.

Measure Boxing Overhead

There are several ways to measure the impact of boxing operations. You can use Performance Monitor to measure the performance impact of boxing overhead on the resource utilization and response times for your application. To do a static analysis of where exactly you are affected by boxing and unboxing in your code, you can analyze MSIL code. Search for box and unbox instructions in MSIL by using the following command line.

Ildasm.exe yourcomponent.dll /text | findstr box
Ildasm.exe yourcomponent.dll /text | findstr unbox

However, you must watch out where exactly you optimize the boxing overhead. The overhead is significant in places where there are frequent iterations such as loops, inserting, and retrieving value types in collections. Instances where boxing occurs only once or twice are not worth optimizing.

Use DirectCast In Your Visual Basic .NET Code

Use the DirectCast operator to cast up and down an inheritance hierarchy instead of using CType. DirectCast offers superior performance because it compiles directly to MSIL. Also, note that DirectCast throws an InvalidCastException if there is no inheritance relationship between two types.

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