Struct Vs Union in c#

In C#, both struct and union are used to create value types, but they are different significantly in memory allocation and usage. ________________________________________ 

1. struct in C# A struct in C# is a value type that contains multiple fields, where each field gets its own memory space. Structs are useful for representing small data structures that do not require reference semantics. 

Memory Allocation in struct Each field inside the struct has its own memory. The size of a struct is the sum of its field sizes (considering padding and alignment). 

Example of struct

Code Snippet

struct Employee { public int Id; // 4 bytes public double Salary; // 8 bytes } class Program { static void Main() { Employee emp; emp.Id = 101; emp.Salary = 50000.50; Console.WriteLine($"Id: {emp.Id}, Salary: {emp.Salary}"); } }

Memory Layout 

Field Type Size Id int 4 bytes Salary double 8 bytes Total 12 bytes (or more with padding) 

When to Use struct? 

✔️ Small, lightweight data structures 

✔️ When no need for inheritance 

✔️ When working with performance-sensitive applications (to avoid heap allocations) ________________________________________

  2. union in C# (Using StructLayout and FieldOffset) C# does not have a direct union like C/C++, but you can create a union-like structure using [StructLayout(LayoutKind.Explicit)] and [FieldOffset] attributes.

  How Does a union Work? 

• All fields share the same memory. 

• The size of the union is equal to the size of the largest field.

 • Writing to one field overwrites the other. Example of union in C#

Code Snippet

using System; using System.Runtime.InteropServices; [StructLayout(LayoutKind.Explicit)] struct MyUnion { [FieldOffset(0)] public int intValue; // 4 bytes [FieldOffset(0)] public float floatValue; // 4 bytes (overlapping memory with intValue) [FieldOffset(0)] public char charValue; // 2 bytes (overlapping with both) } class Program { static void Main() { MyUnion u; u.intValue = 100; Console.WriteLine($"intValue: {u.intValue}, floatValue: {u.floatValue}, charValue: {u.charValue}"); u.floatValue = 9.5f; Console.WriteLine($"intValue: {u.intValue}, floatValue: {u.floatValue}, charValue: {u.charValue}"); u.charValue = 'A'; Console.WriteLine($"intValue: {u.intValue}, floatValue: {u.floatValue}, charValue: {u.charValue}"); } }

Output (may vary due to memory representation) 

intValue: 100, floatValue: 1.401298E-43, charValue: d

intValue: 1091567616, floatValue: 9.5, charValue: e

intValue: 65, floatValue: 9.10844E-44, charValue: A

     Why does this happen?
·        When intValue = 100, the same memory is interpreted as a float and a char.
·        When floatValue = 9.5f, the same memory gets rewritten.
·        When charValue = 'A', it affects the integer and float values.

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Memory Layout of a union

Field	Type	Size (bytes)
intValue	int	4
floatValue	float	4 (shared)
charValue	char	2 (shared within 4 bytes)
Total Memory Used	4 bytes (size of the largest field)

When to Use a union?
✔️ Memory-efficient storage (especially useful for low-level programming)
✔️ Interop with unmanaged C/C++ code (e.g., working with hardware, binary data)
✔️ Working with bitwise operations
🚨 Caution: Accessing a different field than the last written one may produce undefined behavior.

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🚀 Key Differences Between struct and union in C#

Feature	struct	union (via FieldOffset)
Memory Allocation	Each field gets separate memory	All fields share the same memory
Size	Sum of all field sizes (considering alignment)	Size of the largest field
Usage	When fields hold independent values	When only one field is used at a time
Performance	Faster but consumes more memory	Memory-efficient but risky
Common Use Cases	Small objects, lightweight data storage	Interoperability with unmanaged code, memory optimization

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Conclusion
·    Use struct when you need a lightweight data type where each field has its own storage.
·   Use union (via StructLayout + FieldOffset) when you need memory-efficient structures where fields share the same memory.