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A generic struct in C# is a user-defined value type that utilizes one or more type parameters to encapsulate data and behavior independently of specific underlying data types. As a value type, it is allocated on the stack or inline within containing types, and its generic nature ensures compile-time type safety while eliminating the performance overhead of boxing and unboxing operations when working with other value types.

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Syntax Declaration and Instantiation

A generic struct is declared using angle brackets <T> to define type parameters. 
public struct Container<T>
{
    public T Item { get; }

    public Container(T item)
    {
        Item = item;
    }
}

// Instantiation
Container<int> intContainer = new Container<int>(42);
Container<string> stringContainer = new Container<string>("Data");

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Memory Layout and JIT Compilation

The Common Language Runtime (CLR) handles generic structs differently depending on the type argument provided during instantiation:
  • Value Type Arguments: If T is a value type (e.g., int, double, or another struct), the Just-In-Time (JIT) compiler generates a distinct, specialized machine code implementation for that specific type. The memory footprint is calculated exactly based on the size of the inline value type.
  • Reference Type Arguments: If T is a reference type (e.g., string, class), the JIT compiler generates a single shared implementation for all reference types. The struct’s memory footprint will contain a pointer (4 bytes on x86, 8 bytes on x64) to the heap-allocated object.

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Type Constraints

Generic structs support type constraints using the where keyword to restrict the kinds of types that can be substituted for the type parameters. 
public readonly struct DataProcessor<T> where T : struct, IComparable<T>
{
    public T Data { get; }

    public DataProcessor(T data)
    {
        Data = data;
    }
}

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The readonly Modifier

Because structs are copied by value, mutating them can lead to unexpected behavior (torn reads or modifying a copy instead of the original). It is a standard technical practice to mark generic structs as readonly. This forces all fields and properties to be immutable after initialization, allowing the compiler to optimize by passing the struct by in reference without creating defensive copies. 
public readonly struct Point<T> where T : unmanaged
{
    public readonly T X;
    public readonly T Y;

    public Point(T x, T y)
    {
        X = x;
        Y = y;
    }
}

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Default Values and Initialization

When a generic struct is initialized using the default keyword, the CLR zeroes out the memory space allocated for the struct. 
// 'Item' will be 0
Container<int> defaultInt = default; 

// 'Item' will be null
Container<string> defaultString = default;
Note: As of C# 10, generic structs can declare explicit parameterless constructors. Prior to C# 10, parameterless constructors on structs were prohibited by the compiler.

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Structural Limitations

  • Inheritance: Like all structs, generic structs implicitly inherit from System.ValueType (which inherits from System.Object). They cannot explicitly inherit from any class or struct, nor can they be inherited from. They are implicitly sealed.
  • Interface Implementation: Generic structs can implement interfaces. However, casting a generic struct to an interface type will cause a boxing operation, moving the value type to the heap.
  • Self-Referencing: A generic struct cannot contain a field of its own exact type, as the compiler must be able to compute the exact memory size of the struct at compile time.
public struct Node<T>
{
    public T Value;
    // CS0523: Struct member 'Node<T>.Next' of type 'Node<T>' causes a cycle in the struct layout
    public Node<T> Next; 
}
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