Question

I am working on memory mapped files. Is there any way to know the length of memory mapped file content? What I want is to append the existing memory mapped file. Its easy to append the bytes in the file but I am looking to append string.

We can check the CAPACITY property, but it returns the bytes size I think.

To be more clear, I am explaining the scenario. I am creating Memory mapped file A. I write “Hello” when I create it. It works fine. Now I want to write the “World” into existing file A.

I am using below code for this:

 var file = MemoryMappedFile.OpenExisting("myFile");
            string str = "String to append";
            using (var view = file.CreateViewAccessor())
            {
                using (var stream = file.CreateViewStream())
                {
                    System.IO.BinaryWriter writer = new System.IO.BinaryWriter(stream);
                    writer.Write(str.ToArray(), INT SIZE,Convert.ToInt32(view.Capacity), str.Length);//Error
                }
            }
            using (var stream = file.CreateViewStream())
            {
                Console.WriteLine("Reading appended File");
                System.IO.BinaryReader reader = new System.IO.BinaryReader(stream);
                Console.WriteLine(reader.ReadString());
                Console.WriteLine(string.Empty);
            }

I have marked the line with (//) that asks for the size. I want to know the value for (INT SIZE)
Any help would be appreciated.

[EDIT] I am using C#, Visual Studio 2010.

Now I am using this code to append:

var file = MemoryMappedFile.OpenExisting("myFile");
            string str = "String to append";
            string str1 = string.Empty;


            using (var stream = file.CreateViewStream())
            {
                System.IO.BinaryReader reader = new System.IO.BinaryReader(stream);

                str1 = reader.ReadString();
                str1 = str1 + "\n" + str;

                System.IO.BinaryWriter writer = new System.IO.BinaryWriter(stream);
                writer.Write(str1);
            }
            using (var stream = file.CreateViewStream())
            {
                Console.WriteLine("Reading appended File");
                System.IO.BinaryReader reader = new System.IO.BinaryReader(stream);
                Console.WriteLine(reader.ReadString());
                Console.WriteLine(string.Empty);
            }

But its not appending anything. Can you please check my code?

Answer 1

You can’t append to a string in the stream by writing more character after it, as that won’t change the length of the string already written.

Also, there is no method of the BinaryWriter that lets you write to a specific position in the stream like that. For the overload that takes a character array and two integers, the second parameter specifies the position in the array, not the position in the stream.

If you want to change the string written in the stream, you have to read the string from the stream and rewrite the stream with the new string.