MSDN says that you should use structs when you need lightweight objects. Are there any other scenarios when a struct is preferable over a class?
Some people might have forgotten that:
I understand the technical differences between structs and classes, I just don't have a good feel for when to use a struct.
MSDN has the answer: Choosing Between Classes and Structures.
Basically, that page gives you a 4-item checklist and says to use a class unless your type meets all of the criteria.
Do not define a structure unless the type has all of the following characteristics:
- It logically represents a single value, similar to primitive types (integer, double, and so on).
- It has an instance size smaller than 16 bytes.
- It is immutable.
- It will not have to be boxed frequently.
I am surprised I have not read at any of the previous answer this, which I consider the most crucial aspect :
I use structs when I want a type with no identity. For example a 3D point:
public struct ThreeDimensionalPoint
{
public readonly int X, Y, Z;
public ThreeDimensionalPoint(int x, int y, int z)
{
this.X = x;
this.Y = y;
this.Z = z;
}
public override string ToString()
{
return "(X=" + this.X + ", Y=" + this.Y + ", Z=" + this.Z + ")";
}
public override int GetHashCode()
{
return (this.X + 2) ^ (this.Y + 2) ^ (this.Z + 2);
}
public override bool Equals(object obj)
{
if (!(obj is ThreeDimensionalPoint))
return false;
ThreeDimensionalPoint other = (ThreeDimensionalPoint)obj;
return this == other;
}
public static bool operator ==(ThreeDimensionalPoint p1, ThreeDimensionalPoint p2)
{
return p1.X == p2.X && p1.Y == p2.Y && p1.Z == p2.Z;
}
public static bool operator !=(ThreeDimensionalPoint p1, ThreeDimensionalPoint p2)
{
return !(p1 == p2);
}
}
If you have two instances of this struct you don't care if they are a single piece of data in memory or two. You just care about the value(s) they hold.
Use a struct when you want value-type semantics instead of reference-type. Structs are copy-by-value so be careful!
Also see previous questions, e.g.
I would use structs when:
an object is supposed to be read only(every time you pass/assign a struct it gets copied). Read only objects are great when it comes to multithreaded processing as they don't requite locking in most cases.
an object is small and short-living. In such a case there is a good chance that the object will be allocated on the stack which is much more efficient than putting it on the managed heap. What is more the memory allocated by the object will be freed as soon as it goes outside its scope. In other words it's less work for Garbage Collector and the memory is used more efficient.
It is an old topic, but wanted to provide a simple benchmark test.
I have created two .cs files:
public class TestClass
{
public long ID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
and
public struct TestStruct
{
public long ID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
}
Run benchmark:
Results:
BenchmarkDotNet=v0.12.0, OS=Windows 10.0.18362
Intel Core i5-8250U CPU 1.60GHz (Kaby Lake R), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.1.101
[Host] : .NET Core 3.1.1 (CoreCLR 4.700.19.60701, CoreFX 4.700.19.60801), X64 RyuJIT [AttachedDebugger]
DefaultJob : .NET Core 3.1.1 (CoreCLR 4.700.19.60701, CoreFX 4.700.19.60801), X64 RyuJIT
| Method | Mean | Error | StdDev | Ratio | RatioSD | Rank | Gen 0 | Gen 1 | Gen 2 | Allocated |
|--------------- |---------------:|--------------:|--------------:|----------:|--------:|-----:|---------:|------:|------:|----------:|
| UseStruct | 0.0000 ns | 0.0000 ns | 0.0000 ns | 0.000 | 0.00 | 1 | - | - | - | - |
| UseClass | 8.1425 ns | 0.1873 ns | 0.1839 ns | 1.000 | 0.00 | 2 | 0.0127 | - | - | 40 B |
| Use100Struct | 36.9359 ns | 0.4026 ns | 0.3569 ns | 4.548 | 0.12 | 3 | - | - | - | - |
| Use100Class | 759.3495 ns | 14.8029 ns | 17.0471 ns | 93.144 | 3.24 | 4 | 1.2751 | - | - | 4000 B |
| Use10000Struct | 3,002.1976 ns | 25.4853 ns | 22.5920 ns | 369.664 | 8.91 | 5 | - | - | - | - |
| Use10000Class | 76,529.2751 ns | 1,570.9425 ns | 2,667.5795 ns | 9,440.182 | 346.76 | 6 | 127.4414 | - | - | 400000 B |
I have always used a struct when I wanted to group together a few values for passing things back from a method call, but I won't need to use it for anything after I have read those values. Just as a way to keep things clean. I tend to view things in a struct as "throwaway" and things in a class as more useful and "functional"
when you don't really need behavior, but you need more structure than a simple array or dictionary.
Follow up This is how I think of structs in general. I know they can have methods, but I like keeping that overall mental distinction.
As @Simon said, structs provide "value-type" semantics so if you need similar behavior to a built-in data type, use a struct. Since structs are passed by copy you want to make sure they are small in size, about 16 bytes.
Hmm...
I wouldn't use garbage collection as an argument for/against the use of structs vs classes. The managed heap works much like a stack - creating an object just puts it at the top of the heap, which is almost as fast as allocating on the stack. Additionally, if an object is short-lived and does not survive a GC cycle, deallocation is free as the GC only works with memory that's still accessible. (Search MSDN, there's a series of articles on .NET memory management, I'm just too lazy to go dig for them).
Most of the time I use a struct, I wind up kicking myself for doing so, because I later discover that having reference semantics would have made things a bit simpler.
Anyway, those four points in the MSDN article posted above seems a good guideline.
✔️ CONSIDER defining a struct instead of a class if instances of the type are small and commonly short-lived or are commonly embedded in other objects.
