Replace part of a filename in C# - string

Question

I have a folder with .pdf files. In the names of most files I want to replace specific string with another string.
Here's what I've written.
private void btnGetFiles_Click(object sender, EventArgs e)
{
string dir = tbGetFIles.Text;
List<string> FileNames = new List<string>();
DirectoryInfo DirInfo = new DirectoryInfo(dir);
foreach (FileInfo File in DirInfo.GetFiles())
{
FileNames.Add(File.Name);
}
lbFileNames.DataSource = FileNames;
}
Here I extract all file names in List Box.
private void btnReplace_Click(object sender, EventArgs e)
{
string strReplace = tbReplace.Text; // The existing string
string strWith = tbWith.Text; // The new string
string dir = tbGetFIles.Text;
DirectoryInfo DirInfo = new DirectoryInfo(dir);
FileInfo[] names = DirInfo.GetFiles();
foreach (FileInfo f in names)
{
if(f.Name.Contains(strReplace))
{
f.Name.Replace(strReplace, strWith);
}
}
And here I want to do the replacing, but something is going wrong. What?

Answer 1

It sounds like you want to change the name of the file on disk. If so then you need to use the File.Move API vs. changing the actual string which is the file name.
One other mistake you are making is the Replace call itself. A string in .Net is immutable and hence all of the mutating APIs like Replace return a new string vs. changing the old one in place. To see the change you need to assign the new value back to a variable
string newName = f.Name.Replace(strReplace, strWith);
File.Move(f.Name, newName);

Answer 2

f.Name is a read-only property. f.Name.Replace(..) simply returns a new string with the filename you want, but never actually changes the file.
I suggest something along the following, though I haven't tested it:
File.Move(f.Name, f.Name.Replace(strReplace, strWith));

Answer 3

Replace return another string, it doesn't change the original string.
So you need to write
string newName = f.Name.Replace(strReplace, strWith);
of course this doesn't change the name of the file on disk.
If that was your intention then you should look at
File.Move(f.Name, newName);
also keep in mind that File.Move will fail with an exception if the destination file exists.
See here for an example

Answer 4

At a first glance, seems like you're not reassigning the replaced string to your f.Name variable. Try this:
string NewFileName = f.Name.Replace(strReplace, strWith);
File.Copy(f.Name, NewFileName);
File.Delete(f.Name);

Answer 5

When you call string.Replace this doesn't alter your existing string. Instead it is returning a new string.
You need to change your code to something like this:
if(f.Name.Contains(strReplace))
{
string newFileName = f.Name.Replace(strReplace, strWith);
//and work here with your new string
}