Asp.net – Why can't I have properties called "id" and "set_id" in the same class?

I would recommend using JsonPropertyName

For example:

using System.Text.Json.Serialization;

    namespace ConsoleApp1
    {
        public class Class1
        {
            public string id { get; set; }
            [JsonPropertyName("set_id")]
            public string setId { get; set; }
        }
    }

this attribute based on your json library , if you use newtonsoft it would be JsonProperty

Lets findout why we can’t have a 2 variable with names id and set_id together

For understanding this we are going to see IL generated code by c# compiler

for class with 1 property id generated IL looks like this :

.class public auto ansi beforefieldinit ConsoleApp1.Test
    extends [System.Runtime]System.Object
{
    .custom instance void System.Runtime.CompilerServices.NullableContextAttribute::.ctor(uint8) = (
        01 00 01 00 00
    )
    .custom instance void System.Runtime.CompilerServices.NullableAttribute::.ctor(uint8) = (
        01 00 00 00 00
    )
    // Fields
    .field private string '<id>k__BackingField'
    .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
        01 00 00 00
    )
    .custom instance void [System.Runtime]System.Diagnostics.DebuggerBrowsableAttribute::.ctor(valuetype [System.Runtime]System.Diagnostics.DebuggerBrowsableState) = (
        01 00 00 00 00 00 00 00
    )

    // Methods
    .method public hidebysig specialname 
        instance string **get_id** () cil managed 
    {
        .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
            01 00 00 00
        )
        // Method begins at RVA 0x20b8
        // Header size: 1
        // Code size: 7 (0x7)
        .maxstack 8

        IL_0000: ldarg.0
        IL_0001: ldfld string ConsoleApp1.Test::'<id>k__BackingField'
        IL_0006: ret
    } // end of method Test::get_id

    .method public hidebysig specialname 
        instance void set_id (
            string 'value'
        ) cil managed 
    {
        .custom instance void [System.Runtime]System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = (
            01 00 00 00
        )
        // Method begins at RVA 0x20c0
        // Header size: 1
        // Code size: 8 (0x8)
        .maxstack 8

        IL_0000: ldarg.0
        IL_0001: ldarg.1
        IL_0002: stfld string ConsoleApp1.Test::'<id>k__BackingField'
        IL_0007: ret
    } // end of method Test::set_id

    .method public hidebysig specialname rtspecialname 
        instance void .ctor () cil managed 
    {
        // Method begins at RVA 0x20c9
        // Header size: 1
        // Code size: 8 (0x8)
        .maxstack 8

        IL_0000: ldarg.0
        IL_0001: call instance void [System.Runtime]System.Object::.ctor()
        IL_0006: nop
        IL_0007: ret
    } // end of method Test::.ctor

    // Properties
    .property instance string id()
    {
        .get instance string ConsoleApp1.Test::get_id()
        .set instance void ConsoleApp1.Test::set_id(string)
    }

} // end of class ConsoleApp1.Test

In this code you can see getter and setter method are transformed to some 2 methods named set_id and get_id (set_id = set , get_id = get)

Now we have set_id and get_id reserved so we can’t have other field with name set_id or get_id