TextBox on a form is acting weird

I had a main form. I created a child form that is instantiated by this parent form. I have some textbox, buttons and a progress bar on that child form. When I create that form and show it, the textbox's in that form (child) becomes kind of transparent. When I click on them to write something, my child form becomes inactive and the window behind becomes active. Its also transparent and I can see the background. It's like the textboxes have created holes in that form.

Other controls on that form are working fine.

Here is the code I wrote to show the child form

TransferFile_Form sendFile = new TransferFile开发者_StackOverflow社区_Form();            
sendFile.Show(this);

and here is the codebehind for one of those textboxes:

this.txt_file.Location = new System.Drawing.Point(139, 45);
this.txt_file.Name = "txt_file";
this.txt_file.Size = new System.Drawing.Size(320, 21);
this.txt_file.TabIndex = 2;

There is nothing else I am doing. When I used tabs, I can select the textbox and write on it.

Anyone aware of what might have gone wrong?

Did you happen to set the TransparencyKey property of the form to the same color as the background color of the TextBox controls? That would make anything on the form having that color to be transparent and behave as if there was a "hole" there, like you describe.