Common Type System—Field Initialization

Visual C# Tutorials

.NET Framework Tutorials

Common Type System Type Systems Intro § Types and Objects § Type Unification § Reference, Value Types § Accessibility, Visibility § Type Members § Fields § Methods § Locals Overloading Argument Passing Style Pass-by-Value and Pass-by-Reference Output Parameters Managed Pointers Variable Argument Methods Methods, Subclassing Virtuals, Overriding New Slots Exception Handlers Constructors, Instantiation § Default Constructors Value Construction Constructor Chaining Field Initialization Type Initializers Unhandled Exceptions Properties § Indexers Mixed Mode Accessibility Events Operator Overloading Coercion Subclassing, Polymorphism Styles of Inheritance § Implementation Inheritance Interface Inheritance Abstract Classes Interfaces § Method Dispatch Multiple Inheritance Private Interface Inheritance Private Inheritance Accessibility Interface Reimplementation Abstract and Interface? Sealing Types and Methods Runtime Type Checking Namespaces: Organizing Types § Define a Namespace Namespace Resolution Special Types § Delegates § CTS Support Co- and Contravariance Anonymous Delegates Custom Attributes Enumerations § Flags-Style Limitations Helper Methods Generics § Further Reading §

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Field Initialization (Language Feature)

If you use inline initialization for instance fields, the code for that initialization ends up being copied by the C# compiler to all constructors, which don’t delegate to another constructor on the type using the this keyword. For example, consider this type:

class FieldInitExample
{
   int x = 5;
   int y;
 
   public FieldInitExample() : this(5)
   {
   }
 
   public FieldInitExample(int y)
   {
      this.y = y;
   }
}

The int x = 5 line is actually copied to the first few statements of the constructor, which takes an integer argument. It doesn’t get copied to the default constructor because that constructor just delegates to the other overload, which would lead to redundant stores to the x field. The resulting IL is as if you wrote:

class FieldInitExample
{
   int x;
   int y;
 
   public FieldInitExample() : this(5)
   {
   }
 
   public FieldInitExample(int y)
   {
      this.x = 5;
      this.y = y;
   }
}

Static field initialization is handled nearly identically, with the caveat that static class constructors are used instead of instance constructors. The CctorExample type from above could have been written as follows, while keeping the same semantics as before:

class CctorExample
{
   static DateTime classLoadTimestamp = DateTime.Now;
}