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Created attachment 25983 [details]
minimum code containing the bug
For an application I'm developing, I need the user to be able to open large numbers of selected files (a typical use case involves around 100 to a couple of thousand files). To do so, I use an OpenFileDialog. When selecting up to around 80 files (shift-click, control-click or control-a), the time it takes to select these files (the time it takes for them them turn blue and show in the filename textbox, measured from the moment you click on a file or press control-a) remains more or less constant. However, when selecting more than 80 files, this time increases very fast, so that it eventually takes many minutes to select a few hundred files. On Windows using the .NET framework, selecting the same amount of files is almost instant.
OS: Ubuntu 16.04
Mono version: 220.127.116.11
To compile the attached file: mcs -pkg:dotnet Program.cs
To run: mono Program.exe
Expected behaviour: OpenFileDialog should be able to select large numbers of files without any excessive delay, as is the case in Windows .NET.
Experienced behaviour: When selecting more than around 80 files, OpenFileDialog takes an excessive amount of time, from about 10 seconds to many minutes depending on how many files are selected, during which the UI is unresponsive.