Records
Records can be stored on a heap or a stack:
// heap, like a classrecord Person(string Name);
// stack, like a structrecord struct Person2(string Name);Depending on the choice as above, the compiler will transform the record into either a class or a struct (in a process called the lowering). Record is just s “sugar syntax” in .NET.
Records are useful when we’re dealing with classes that just carry information and do not have any logic/methods (DTOs).
Features
Printing
Printing an instance of a record by default prints its content. An instance of a class would print its type.
Console.WriteLine(recordInstance); // Person { Name = "Marcin" }
Console.WriteLine(classInstance); // Namespace.ClassNameRecords printing behavior may be overridden just like in a normal class.
Equality
Class instances (unless explicitly coded otherwise) will not be equal even if all properties have the same values. Equality is checked by reference.
In the case of records, an equality check compares the values of the properties.
The with operator
Records may be copied (by value) with some changes to original values like this:
var rec1 = new Person("Marcin", 25);var rec2 = rec1 with { Age = 20 }; // only age gets modified in the new record instanceDeconstructing
A bit similarly to JS, we can extract some values from records:
(var name, var age) = rec1;Intermediate Language
To better understand the difference between a class and a struct, let’s compare IL generated for an empty class and record.
Class
public class SimpleClass{}.class public auto ansi beforefieldinit HelloWorld.SimpleClass extends [System.Runtime]System.Object{
.method public hidebysig specialname rtspecialname instance void .ctor() cil managed { .maxstack 8
IL_0000: ldarg.0 // this IL_0001: call instance void [System.Runtime]System.Object::.ctor() IL_0006: nop IL_0007: ret
} // end of method SimpleClass::.ctor} // end of class HelloWorld.SimpleClassThe generated IL is really simple, and it really just corresponds to the provided C# code. The only thing added is a constructor.
Record
public record SimpleRecord();.class public auto ansi beforefieldinit HelloWorld.SimpleRecord extends [System.Runtime]System.Object implements class [System.Runtime]System.IEquatable`1<class HelloWorld.SimpleRecord>{ .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(unsigned int8) = (01 00 01 00 00 ) // ..... // unsigned int8(1) // 0x01 .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(unsigned int8) = (01 00 00 00 00 ) // ..... // unsigned int8(0) // 0x00 .interfaceimpl type class [System.Runtime]System.IEquatable`1<class HelloWorld.SimpleRecord> .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(unsigned int8) = (01 00 00 00 00 ) // ..... // unsigned int8(0) // 0x00
.method public hidebysig specialname rtspecialname instance void .ctor() cil managed { .maxstack 8
// [3 1 - 3 30] IL_0000: ldarg.0 // this IL_0001: call instance void [System.Runtime]System.Object::.ctor() IL_0006: nop IL_0007: ret
} // end of method SimpleRecord::.ctor
.method family hidebysig virtual newslot specialname instance class [System.Runtime]System.Type get_EqualityContract() cil managed { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldtoken HelloWorld.SimpleRecord IL_0005: call class [System.Runtime]System.Type [System.Runtime]System.Type::GetTypeFromHandle(valuetype [System.Runtime]System.RuntimeTypeHandle) IL_000a: ret
} // end of method SimpleRecord::get_EqualityContract
.method public hidebysig virtual instance string ToString() cil managed { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 2 .locals init ( [0] class [System.Runtime]System.Text.StringBuilder V_0 )
IL_0000: newobj instance void [System.Runtime]System.Text.StringBuilder::.ctor() IL_0005: stloc.0 // V_0 IL_0006: ldloc.0 // V_0 IL_0007: ldstr "SimpleRecord" IL_000c: callvirt instance class [System.Runtime]System.Text.StringBuilder [System.Runtime]System.Text.StringBuilder::Append(string) IL_0011: pop IL_0012: ldloc.0 // V_0 IL_0013: ldstr " { " IL_0018: callvirt instance class [System.Runtime]System.Text.StringBuilder [System.Runtime]System.Text.StringBuilder::Append(string) IL_001d: pop IL_001e: ldarg.0 // this IL_001f: ldloc.0 // V_0 IL_0020: callvirt instance bool HelloWorld.SimpleRecord::PrintMembers(class [System.Runtime]System.Text.StringBuilder) IL_0025: brfalse.s IL_0030 IL_0027: ldloc.0 // V_0 IL_0028: ldc.i4.s 32 // 0x20 IL_002a: callvirt instance class [System.Runtime]System.Text.StringBuilder [System.Runtime]System.Text.StringBuilder::Append(char) IL_002f: pop IL_0030: ldloc.0 // V_0 IL_0031: ldc.i4.s 125 // 0x7d IL_0033: callvirt instance class [System.Runtime]System.Text.StringBuilder [System.Runtime]System.Text.StringBuilder::Append(char) IL_0038: pop IL_0039: ldloc.0 // V_0 IL_003a: callvirt instance string [System.Runtime]System.Object::ToString() IL_003f: ret
} // end of method SimpleRecord::ToString
.method family hidebysig virtual newslot instance bool PrintMembers( class [System.Runtime]System.Text.StringBuilder builder ) cil managed { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldc.i4.0 IL_0001: ret
} // end of method SimpleRecord::PrintMembers
.method public hidebysig static specialname bool op_Inequality( class HelloWorld.SimpleRecord left, class HelloWorld.SimpleRecord right ) cil managed { .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(unsigned int8) = (01 00 02 00 00 ) // ..... // unsigned int8(2) // 0x02 .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldarg.0 // left IL_0001: ldarg.1 // right IL_0002: call bool HelloWorld.SimpleRecord::op_Equality(class HelloWorld.SimpleRecord, class HelloWorld.SimpleRecord) IL_0007: ldc.i4.0 IL_0008: ceq IL_000a: ret
} // end of method SimpleRecord::op_Inequality
.method public hidebysig static specialname bool op_Equality( class HelloWorld.SimpleRecord left, class HelloWorld.SimpleRecord right ) cil managed { .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(unsigned int8) = (01 00 02 00 00 ) // ..... // unsigned int8(2) // 0x02 .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldarg.0 // left IL_0001: ldarg.1 // right IL_0002: beq.s IL_0013 IL_0004: ldarg.0 // left IL_0005: brfalse.s IL_0010 IL_0007: ldarg.0 // left IL_0008: ldarg.1 // right IL_0009: callvirt instance bool HelloWorld.SimpleRecord::Equals(class HelloWorld.SimpleRecord) IL_000e: br.s IL_0011 IL_0010: ldc.i4.0 IL_0011: br.s IL_0014 IL_0013: ldc.i4.1 IL_0014: ret
} // end of method SimpleRecord::op_Equality
.method public hidebysig virtual instance int32 GetHashCode() cil managed { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: call class [System.Collections]System.Collections.Generic.EqualityComparer`1<!0/*class [System.Runtime]System.Type*/> class [System.Collections]System.Collections.Generic.EqualityComparer`1<class [System.Runtime]System.Type>::get_Default() IL_0005: ldarg.0 // this IL_0006: callvirt instance class [System.Runtime]System.Type HelloWorld.SimpleRecord::get_EqualityContract() IL_000b: callvirt instance int32 class [System.Collections]System.Collections.Generic.EqualityComparer`1<class [System.Runtime]System.Type>::GetHashCode(!0/*class [System.Runtime]System.Type*/) IL_0010: ret
} // end of method SimpleRecord::GetHashCode
.method public hidebysig virtual instance bool Equals( object obj ) cil managed { .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(unsigned int8) = (01 00 02 00 00 ) // ..... // unsigned int8(2) // 0x02 .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldarg.0 // this IL_0001: ldarg.1 // obj IL_0002: isinst HelloWorld.SimpleRecord IL_0007: callvirt instance bool HelloWorld.SimpleRecord::Equals(class HelloWorld.SimpleRecord) IL_000c: ret
} // end of method SimpleRecord::Equals
.method public hidebysig virtual newslot instance bool Equals( class HelloWorld.SimpleRecord other ) cil managed { .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(unsigned int8) = (01 00 02 00 00 ) // ..... // unsigned int8(2) // 0x02 .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldarg.0 // this IL_0001: ldarg.1 // other IL_0002: beq.s IL_001d IL_0004: ldarg.1 // other IL_0005: brfalse.s IL_001a IL_0007: ldarg.0 // this IL_0008: callvirt instance class [System.Runtime]System.Type HelloWorld.SimpleRecord::get_EqualityContract() IL_000d: ldarg.1 // other IL_000e: callvirt instance class [System.Runtime]System.Type HelloWorld.SimpleRecord::get_EqualityContract() IL_0013: call bool [System.Runtime]System.Type::op_Equality(class [System.Runtime]System.Type, class [System.Runtime]System.Type) IL_0018: br.s IL_001b IL_001a: ldc.i4.0 IL_001b: br.s IL_001e IL_001d: ldc.i4.1 IL_001e: ret
} // end of method SimpleRecord::Equals
.method public hidebysig virtual newslot instance class HelloWorld.SimpleRecord '<Clone>$'() cil managed { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldarg.0 // this IL_0001: newobj instance void HelloWorld.SimpleRecord::.ctor(class HelloWorld.SimpleRecord) IL_0006: ret
} // end of method SimpleRecord::'<Clone>$'
.method family hidebysig specialname rtspecialname instance void .ctor( class HelloWorld.SimpleRecord original ) cil managed { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .maxstack 8
IL_0000: ldarg.0 // this IL_0001: call instance void [System.Runtime]System.Object::.ctor() IL_0006: nop IL_0007: ret
} // end of method SimpleRecord::.ctor
.property instance class [System.Runtime]System.Type EqualityContract() { .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (01 00 00 00 ) .get instance class [System.Runtime]System.Type HelloWorld.SimpleRecord::get_EqualityContract() } // end of property SimpleRecord::EqualityContract} // end of class HelloWorld.SimpleRecordThat’s a huge difference. An empty record results in lots of code being generated. All this code is a result of features that records support:
- equality logic (you can see a few equality-related methods)
ToStringoverride - interesting point is how the{character is added viaAppend(string)override ofStringBuilder, while the closing bracket (}) usesAppend(char)(with value 125 representing})- cloning