What's the difference between struct and class in .NET?

Question

What\'s the difference between struct and class in .NET?

Answer 1

There is one interesting case of "class vs struct" puzzle - situation when you need to return several results from the method: choose which to use. If you know the ValueTuple story - you know that ValueTuple (struct) was added because it should be more effective then Tuple (class). But what does it mean in numbers? Two tests: one is struct/class that have 2 fields, other with struct/class that have 8 fields (with dimension more then 4 - class should become more effective then struct in terms processor ticks, but of course GC load also should be considered).

P.S. Another benchmark for specific case 'sturct or class with collections' is there: https://stackoverflow.com/a/45276657/506147

BenchmarkDotNet=v0.10.10, OS=Windows 10 Redstone 2 [1703, Creators Update] (10.0.15063.726)
Processor=Intel Core i5-2500K CPU 3.30GHz (Sandy Bridge), ProcessorCount=4
Frequency=3233540 Hz, Resolution=309.2586 ns, Timer=TSC
.NET Core SDK=2.0.3
  [Host] : .NET Core 2.0.3 (Framework 4.6.25815.02), 64bit RyuJIT
  Clr    : .NET Framework 4.7 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.7.2115.0
  Core   : .NET Core 2.0.3 (Framework 4.6.25815.02), 64bit RyuJIT


            Method |  Job | Runtime |     Mean |     Error |    StdDev |      Min |      Max |   Median | Rank |  Gen 0 | Allocated |
------------------ |----- |-------- |---------:|----------:|----------:|---------:|---------:|---------:|-----:|-------:|----------:|
  TestStructReturn |  Clr |     Clr | 17.57 ns | 0.1960 ns | 0.1834 ns | 17.25 ns | 17.89 ns | 17.55 ns |    4 | 0.0127 |      40 B |
   TestClassReturn |  Clr |     Clr | 21.93 ns | 0.4554 ns | 0.5244 ns | 21.17 ns | 23.26 ns | 21.86 ns |    5 | 0.0229 |      72 B |
 TestStructReturn8 |  Clr |     Clr | 38.99 ns | 0.8302 ns | 1.4097 ns | 37.36 ns | 42.35 ns | 38.50 ns |    8 | 0.0127 |      40 B |
  TestClassReturn8 |  Clr |     Clr | 23.69 ns | 0.5373 ns | 0.6987 ns | 22.70 ns | 25.24 ns | 23.37 ns |    6 | 0.0305 |      96 B |
  TestStructReturn | Core |    Core | 12.28 ns | 0.1882 ns | 0.1760 ns | 11.92 ns | 12.57 ns | 12.30 ns |    1 | 0.0127 |      40 B |
   TestClassReturn | Core |    Core | 15.33 ns | 0.4343 ns | 0.4063 ns | 14.83 ns | 16.44 ns | 15.31 ns |    2 | 0.0229 |      72 B |
 TestStructReturn8 | Core |    Core | 34.11 ns | 0.7089 ns | 1.4954 ns | 31.52 ns | 36.81 ns | 34.03 ns |    7 | 0.0127 |      40 B |
  TestClassReturn8 | Core |    Core | 17.04 ns | 0.2299 ns | 0.2150 ns | 16.68 ns | 17.41 ns | 16.98 ns |    3 | 0.0305 |      96 B |

Code test:

using System;
using System.Text;
using System.Collections.Generic;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Attributes.Columns;
using BenchmarkDotNet.Attributes.Exporters;
using BenchmarkDotNet.Attributes.Jobs;
using DashboardCode.Routines.Json;

namespace Benchmark
{
    //[Config(typeof(MyManualConfig))]
    [RankColumn, MinColumn, MaxColumn, StdDevColumn, MedianColumn]
    [ClrJob, CoreJob]
    [HtmlExporter, MarkdownExporter]
    [MemoryDiagnoser]
    public class BenchmarkStructOrClass
    {
        static TestStruct testStruct = new TestStruct();
        static TestClass testClass = new TestClass();
        static TestStruct8 testStruct8 = new TestStruct8();
        static TestClass8 testClass8 = new TestClass8();
        [Benchmark]
        public void TestStructReturn()
        {
            testStruct.TestMethod();
        }

        [Benchmark]
        public void TestClassReturn()
        {
            testClass.TestMethod();
        }


        [Benchmark]
        public void TestStructReturn8()
        {
            testStruct8.TestMethod();
        }

        [Benchmark]
        public void TestClassReturn8()
        {
            testClass8.TestMethod();
        }

        public class TestStruct
        {
            public int Number = 5;
            public struct StructType
            {
                public T Instance;
                public List List;
            }

            public int TestMethod()
            {
                var s = Method1(1);
                return s.Instance;
            }

            private StructType Method1(int i)
            {
                return Method2(++i);
            }

            private StructType Method2(int i)
            {
                return Method3(++i);
            }

            private StructType Method3(int i)
            {
                return Method4(++i);
            }

            private StructType Method4(int i)
            {
                var x = new StructType();
                x.List = new List();
                x.Instance = ++i;
                return x;
            }
        }

        public class TestClass
        {
            public int Number = 5;
            public class ClassType
            {
                public T Instance;
                public List List;
            }

            public int TestMethod()
            {
                var s = Method1(1);
                return s.Instance;
            }

            private ClassType Method1(int i)
            {
                return Method2(++i);
            }

            private ClassType Method2(int i)
            {
                return Method3(++i);
            }

            private ClassType Method3(int i)
            {
                return Method4(++i);
            }

            private ClassType Method4(int i)
            {
                var x = new ClassType();
                x.List = new List();
                x.Instance = ++i;
                return x;
            }
        }

        public class TestStruct8
        {
            public int Number = 5;
            public struct StructType
            {
                public T Instance1;
                public T Instance2;
                public T Instance3;
                public T Instance4;
                public T Instance5;
                public T Instance6;
                public T Instance7;
                public List List;
            }

            public int TestMethod()
            {
                var s = Method1(1);
                return s.Instance1;
            }

            private StructType Method1(int i)
            {
                return Method2(++i);
            }

            private StructType Method2(int i)
            {
                return Method3(++i);
            }

            private StructType Method3(int i)
            {
                return Method4(++i);
            }

            private StructType Method4(int i)
            {
                var x = new StructType();
                x.List = new List();
                x.Instance1 = ++i;
                return x;
            }
        }

        public class TestClass8
        {
            public int Number = 5;
            public class ClassType
            {
                public T Instance1;
                public T Instance2;
                public T Instance3;
                public T Instance4;
                public T Instance5;
                public T Instance6;
                public T Instance7;
                public List List;
            }

            public int TestMethod()
            {
                var s = Method1(1);
                return s.Instance1;
            }

            private ClassType Method1(int i)
            {
                return Method2(++i);
            }

            private ClassType Method2(int i)
            {
                return Method3(++i);
            }

            private ClassType Method3(int i)
            {
                return Method4(++i);
            }

            private ClassType Method4(int i)
            {
                var x = new ClassType();
                x.List = new List();
                x.Instance1 = ++i;
                return x;
            }
        }
    }
}