If you use auto-implemented properties, the compiler adds the readonly modifier to the get accessor for read-write properties. The compiler adds the readonly modifier to the auto-implemented property declarations for properties with only a get accessor.

Properties with init-only setter are considered readonly/immutable. If they are autoimplemented, compiler generates readonly modifier.

 object Obj { get; init; }

.method /* 060000D7 */ private hidebysig specialname instance object get_Obj () cil managed 
{
	.custom instance void [System.Runtime]System.Runtime.CompilerServices.IsReadOnlyAttribute::.ctor() = ( 01 00 00 00 )
	.custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 )
	// Method begins at RVA 0x35a0
	// Code size 7 (0x7)
	.maxstack 8

…

System.Runtime.CompilerServices.IsReadOnlyAttribute expresses readonly modifier.

Usually by readonly property is meant property without any setter. So it is better to explicitly denote that autogenerated RW-, readonly- and with init-only-setter- properties get readonly modifier to getter.

Document Details

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• ID: ac8804c7-6acd-ab2a-4e14-0b99265969a5
• Version Independent ID: 183efcbd-a781-d303-f9cd-1c22da1676de
• Content: Write safe and efficient C# code
• Content Source: docs/csharp/write-safe-efficient-code.md
• Product: dotnet-csharp
• Technology: csharp-advanced-concepts
• GitHub Login: @BillWagner
• Microsoft Alias: wiwagn

Associated WorkItem - 54742