Step 1: Click New Project, then select Visual C# on the left, then Windows and then select Windows Forms Application. Name your project "ReadTxtFile" and then click OK
Step 2: Design your form as below

Step 3: Add code to button click event handler as below
private async void btnOpen_Click(object sender, EventArgs e)
{
try
{
//Open file dialog, allows you to select a txt file
using (OpenFileDialog ofd = new OpenFileDialog() { Filter = "Text Documents|*.txt", Multiselect = false, ValidateNames = true })
{
if (ofd.ShowDialog() == DialogResult.OK)
{
using (StreamReader sr = new StreamReader(ofd.FileName))
{
txtValue.Text = await sr.ReadToEndAsync();
}
}
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message, "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);
}
}
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