Barcod scanning event in textbox

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C#4.0

I have developed a point of sale application for a retail store in the c# winfrom application.
now my question is my application has a barcode scanning feature like the system will check either the product quantity available in the stock or not by scanning a barcode.
I will set to the textbox text change event my system will first check the barcode is exist in the database or not if the barcode does not exist in the database then my application shows the barcode not existing message.
now the problem is that.
once the user starts the scanning of the barcode application will display the message at the entering of each character in the textbox box I put a condition on the textbox text change event.
please, anyone, give me an idea,
how can I run my function once the barcode scanning is finished.
Note: all my products have different length barcode-like some products have barcodes of 5 characters some products have barcodes of 10 characters.

What I have tried:

private void searchproductnamemetroTextBox_TextChanged(object sender, EventArgs e)
{
    if(Barcodeexist())
    {
      //code of fetch details
    }
    else
    {
     Messagebox.show("Barcode not exist")
    }
}