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Language Feature Highlight: Local Type Inference in C# 3.0 and Visual Basic 9.0

  • November 14, 2007
  • By Mark Strawmyer
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The focus of this article will be on highlighting the local type inference feature that has been added to C# 3.0 and Visual Basic 9.0 languages. You'll touch on what it is, the syntax behind it, and why it is relevant to understand. You'll also touch on some examples of invalid uses because it can be just as helpful to examine what it is not to get a grasp on the concept.

Local Type Inference Defined

Local type inference is a language feature that allows you to define variables and use them without worrying about their true type. Local type inference is also interchangeably known as implicitly typed local variables. The burden is put on the respective language compiler to determine the type of a variable by inferring it from the expression assigned to the variable. The result is type safety while allowing you to write more relaxed code, which is required to support Language Integrated Query (LINQ).

Based on the description and a first glance of code, it is very easy to mistake type inference to be similar to defining everything as a type object or use of variants, which is heavily used in Visual Basic 6.0. This is entirely untrue and not what type inference is about. Type inferred variables are strongly typed. The type cannot be changed once it is assigned as could be done with a variant type so it does not involve any casting operations or the resulting performance implications. A strong type is assigned, but simply done so by the compiler based on the results of the expression assigned to the variable. The net effect is the true type isn't as readily apparent when reading code, but the Visual Studio IDE will tell you the type assigned along with the GetType() method will return a strong type at runtime.

There may be temptation over time to get lazy and let the compiler do the work for you by using type inference across the board. However, this is where the local part of local type inference comes into play. Type inference can only be used within a local scope where its type can be inferred by the expression assignment. Type inference cannot be applied to any of the following:

  • Cannot be a part of a member property declaration on a class, struct, or interface
  • Cannot be used in a parameter list on a method
  • Cannot be a return type for a method
  • Cannot be defined without a right hand assignment expression
  • Cannot reassign to be a different type once type has been inferred

Local Type Inference in C# 3.0

C# 3.0 implements local type inference through the var keyword in place of a specific type in a variable declaration.

The sample code below demonstrates the syntax for local type inference in C#. I created a new Windows console project to hold the code. Visual Studio 2008 Beta 2 was used to create the examples contained within.

namespace CodeGuru.TypeInference{   class Program   {      static void Main(string[] args)      {         int a = 5;         var b = a;           // int         var x = 5.5M;        // double         var s = "string";    // string         var l = s.Length;    // int         Console.WriteLine("value of b is {0} and type is {1}",            b, b.GetType().ToString());         Console.WriteLine("type of x is {0}", x.GetType().ToString());         Console.WriteLine("type of s is {0}", s.GetType().ToString());         Console.WriteLine("type of l is {0}", l.GetType().ToString());         Console.ReadLine();      }   })

It can be just as useful at times to look at examples where something does not apply. The following sample C# code demonstrates situations in which local type inference cannot be used. The code that is listed below will result in six different compile errors based on invalid usage and intentionally will not compile.

namespace CodeGuru.TypeInference{   class Program   {      var test = "invalid use";    // invalid in member declaration      // Invalid as parameter      public void TryAsParameter(var parm)      {      }      // Invalid as return type      public var TryAsReturnType()      {         return "invalid use";      }      public void TryInvalidLocalUse()      {         var local1;                // must be initialized         var local2 = null;         // can't infer type from null         var local3 = 5;            // valid use         local3 = "change type";    // can't change type      }   }}

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