在更改之前获取值

Question

I am having trouble with getting a value in a text box before the value is changed either by user input or programmatically.

I figure it has to do with the _TextChanged event but once this executes how do I get the old value which was already there before the change?

For example number 3 is in the text field. The number then changes to 4 how do I save 3 ?

Thanks in advance!

Answer 1

Do not store anything in the TextBox . Use it only to display/edit value .

private string value;

private void textBox1_TextChanged(object sender, EventArgs e)
{
    // at this moment value is still old
    var oldValue = value;
    value = ((TextBox)sender).Text; // text1.Text

    // here you have oldValue and new value
}

Maybe you don't want to update value after each TextChanged . Maybe you will do it when form is closed, some button is pressed, etc.

Anyhow idea should be clear: if you need to have old value, then store it somewhere yourself (as a variable, as a Tag , as application setting, exporting, etc.).

Also, take care about how user will work. If TextBox contains 3 , then user may first press 4 and only then delete 3 . Or user may accidentally press 54 . Which of those will be old value: 3 , 34 , 54 ? Simply to demonstrate you, what there could be consequent problems to having concept of old value . Mayhaps you don't want it and there is better solution to what you are trying to solve by getting old value.

Answer 2

Why not save the current value in the textbox in a variable during the textchanged event. That way, it is the "previous" value when the event is fired again.

Answer 3

You can save the value of textbox in some variable. For example string currentVal = txtName.Text;

string currentVal can be used as a previous value in text_changed event.

Answer 4

Have you tried making a timer. Every tick you write the input to a string. If the input of the textbox is not equal to the content in the string, you trigger what you wanted to do. That way you can trigger a void with the old input.

private string oldInput;

....

void TimerTick(Object source, ElapsedEventArgs e)
{
    if (oldInput != MyTextBox.Text) MyVoid(oldInput);
    oldInput = MyTextBox.Text;
}