Ctrl+Backspace modifies readonly TextBox when AutoCompleteMode and AutoCompleteSource are not None

[Kratohs]

.NET version

net9.0-windows

Did it work in .NET Framework?

Not tested/verified

Issue description

Pressing Ctrl+Backspace in a TextBox that has no selection and has ReadOnly set to true modifies its text as if it were not read-only if AutoCompleteMode and AutoCompleteSource are not None.

Attached is a minimal reproduction. In a more realistic scenario, ReadOnly might be set to true in response to an event, some time after it was initialized as false.

Steps to reproduce

Bug.fsproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net9.0-windows</TargetFramework>
    <UseWindowsForms>true</UseWindowsForms>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Program.fs" />
  </ItemGroup>

</Project>

Program.fs

open System
open System.Windows.Forms

[<EntryPoint; STAThread>]
let main _ =
    let tb =
        new TextBox
            ( AutoCompleteMode = AutoCompleteMode.SuggestAppend
            , AutoCompleteSource = AutoCompleteSource.CustomSource
            , Text = "abc"
            , ReadOnly = true
            )
    let f = new Form()
    f.Controls.Add tb
    f.ShowDialog() |> ignore
    0

dotnet run

Ctrl+Backspace deletes text if there is no selection.

[merriemcgaw]

@Olina-Zhang can your team repro this in C#?

[Zheng-Li01]

The issue can reproduce from .NET 6.0~ latest .NET 10.0 & .NET Framework by using the Ctrl+Backspace keyboard deletes text if there is no selection when the AutoCompleteMode & AutoCompleteSource properties are setting together even though the ReadOnly setting as true. see below reproduce video.

13276.mp4

[Epica3055]

I have been investigating this issue but I failed to bring forward a prefect solution.

maybe there is a workaround you can use:
Before you set ReadObly = true, you can set AutoCompleteMode = AutoCompleteMode.None first, since when it is readonly ,autocomplete function is not usable.

[LeafShi1]

This is a problem with Windows native controls, and it can also be reproduced in MFC projects

@Tanya-Solyanik Do we need to investigate this issue for further?

[Tanya-Solyanik]

Thank you, @LeafShi1 and @Epica3055, closing as external.