Check out example codes for "file picker uwp c#". It will help you in understanding the concepts better.

Code Example 1

public async Task GetFile()
{
    Windows.Storage.Pickers.FileOpenPicker picker = new Windows.Storage.Pickers.FileOpenPicker();
    picker.ViewMode = Windows.Storage.Pickers.PickerViewMode.List;
    picker.FileTypeFilter.Add(".txt");
    //Prompt the user to open the log file:
    Windows.Storage.StorageFile logFile = await picker.PickSingleFileAsync();

    try
    {
        using (var stream = await logFile.OpenStreamForReadAsync())
        using (var reader = new StreamReader(stream))
        {
            var line = await reader.ReadLineAsync();
            Debug.WriteLine($"The first line: {line} - waiting");
            await Task.Delay(10000);
            line = await reader.ReadLineAsync();
            Debug.WriteLine($"The next line: {line} - waiting");
        }
    }
    catch (Exception exc)
    {
        Debug.WriteLine($"Exception {exc.Message}");
    }
}

Code Example 2

// Create the picker object and set options
var openPicker = new Windows.Storage.Pickers.FileOpenPicker();
openPicker.viewMode = Windows.Storage.Pickers.PickerViewMode.thumbnail;
openPicker.suggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.picturesLibrary;
// Users expect to have a filtered view of their folders depending on the scenario.
// For example, when choosing a documents folder, restrict the filetypes to documents for your application.
openPicker.fileTypeFilter.replaceAll([".png", ".jpg", ".jpeg"]);

// Open the picker for the user to pick a file
openPicker.pickSingleFileAsync().then(function (file) {
    if (file) {
        // Application now has read/write access to the picked file
        WinJS.log && WinJS.log("Picked photo: " + file.name, "sample", "status");
    } else {
        // The picker was dismissed with no selected file
        WinJS.log && WinJS.log("Operation cancelled.", "sample", "status");
    }
});

Code Example 3

var picker = new Windows.Storage.Pickers.FileOpenPicker();
picker.ViewMode = Windows.Storage.Pickers.PickerViewMode.Thumbnail;
picker.SuggestedStartLocation = Windows.Storage.Pickers.PickerLocationId.PicturesLibrary;
picker.FileTypeFilter.Add(".jpg");
picker.FileTypeFilter.Add(".jpeg");
picker.FileTypeFilter.Add(".png");

Windows.Storage.StorageFile file = await picker.PickSingleFileAsync();
if (file != null)
{
    // Application now has read/write access to the picked file
    this.textBlock.Text = "Picked photo: " + file.Name;
}
else
{
    this.textBlock.Text = "Operation cancelled.";
}

Code Example 4

FileOpenPicker openPicker = new FileOpenPicker();
openPicker.ViewMode = PickerViewMode.Thumbnail;
openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
openPicker.FileTypeFilter.Add(".jpg");
openPicker.FileTypeFilter.Add(".jpeg");
openPicker.FileTypeFilter.Add(".png");

StorageFile file = await openPicker.PickSingleFileAsync();
if (file != null)
{
    // Application now has read/write access to the picked file
    OutputTextBlock.Text = "Picked photo: " + file.Name;
}
else
{
    OutputTextBlock.Text = "Operation cancelled.";
}

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