String.Replace in C# .NET

Question

I would like to know why it is not working:

string filename = optionFileNameFormat; // "{year}-{month}-{day} {name}"
Dictionary<string, string> tagList = new Dictionary<string, string>();
tagList.Add("author",System.Security.Principal.WindowsIdentity.GetCurrent().Name);
tagList.Add("year" , "" + DateTime.Now.Year);
tagList.Add("month", "" + DateTime.Now.Month);
tagList.Add("day"  , "" + DateTime.Now.Day);

foreach (var property in tagList)
{
    filename.Replace(@"{" + property.Key + @"}", property.Value);
}

I don't have any error, but my string doesn't change.

Joel Coehoorn · Accepted Answer · 2013-09-02 14:38:35Z

12

There may be other problems as well, but what jumped out at me right away is that the Replace() function does not change the string. Instead, it returns a new string. Therefore, you need to assign the result of the function back to the original:

filename = filename.Replace(@"{" + property.Key + @"}", property.Value);

answered Sep 2, 2013 at 14:38

Joel Coehoorn

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1 Comment

Matthew Watson Over a year ago

To elaborate: This is because strings are immutable, so if you ever call a method to change one, what actually happens is that the method returns the changed string and the original string is unmodified.

Peter Mortensen · Accepted Answer · 2020-07-20 18:12:24Z

The String.Replace method returns a new string. It doesn't change the original string.

Returns a new string in which all occurrences of a specified Unicode character or String in the current string are replaced with another specified Unicode character or String

So, you should assign a new string or an existing one inside your foreach loop:

filename = filename.Replace(@"{" + property.Key + @"}", property.Value);

or

string newfilename = filename.Replace(@"{" + property.Key + @"}", property.Value);

And remember, in .NET, strings are immutable types. You can't change them. Even if you think you change them, you create new string objects.

Peter Mortensen · Accepted Answer · 2020-07-20 18:14:31Z

1

In

foreach (var property in tagList)
{
    filename.Replace(@"{" + property.Key + @"}", property.Value);
}

just do the below change:

filename =  filename.Replace(@"{" + property.Key + @"}", property.Value);

edited Jul 20, 2020 at 18:14

Peter Mortensen

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answered Sep 3, 2013 at 4:54

Haidar Abbas

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Comments

Peter Mortensen · Accepted Answer · 2020-07-20 18:15:07Z

1

This is the completed code:

 string filename = optionFileNameFormat; // "{year}-{month}-{day} {name}"
 Dictionary<string, string> tagList = new Dictionary<string, string>();
 tagList.Add("author",System.Security.Principal.WindowsIdentity.GetCurrent().Name);
 tagList.Add("year" , "" + DateTime.Now.Year);
 tagList.Add("month", "" + DateTime.Now.Month);
 tagList.Add("day"  , "" + DateTime.Now.Day);

 foreach (var property in tagList)
 {
     filename= filename.Replace(@"{" + property.Key + @"}", property.Value);
 }

edited Jul 20, 2020 at 18:15

Peter Mortensen

31.4k22 gold badges110 silver badges134 bronze badges

answered Sep 3, 2013 at 5:45

Shyam sundar shah

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