C# - Read a double value

1. Suppose there is only one single double value written into a file in binary format. How can I read that value using C# or Java?

2. If I开发者_StackOverflow have to find a double value from a huge binary file, what techniques should I use to find that?

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Double is 8 bytes. To read single double from binary file you can use BitConverter class:

var fileContent = File.ReadAllBytes("C:\\1.bin");
double value = BitConverter.ToDouble(fileContent, 0);

If you need to read double from the middle of the file, replace 0 with the byte offset.

If you don't know the offset, you can't possibly tell that certain value in the byte array is double, integer or string.

Another approach is:

using (var fileStream = File.OpenRead("C:\\1.bin"))
using (var binaryReader = new BinaryReader(fileStream))
{
    // fileStream.Seek(0, SeekOrigin.Begin); // uncomment this line and set offset if the double is in the middle of the file
    var value = binaryReader.ReadDouble();
}

The second approach is better for big files, as it does not load whole file content to the memory.

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You can use the BinaryReader class.

double value;
using( Stream stream = File.OpenRead(fileName) )
using( BinaryReader reader = new BinaryReader(stream) )
{
    value = reader.ReadDouble();
}

For the second point, if you know the offset, simply use the Stream.Seek method.

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It seems that we would need to know how the double value was encoded in the file before we could go about finding it.

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1)

        double theDouble;
        using (Stream sr = new FileStream(@"C:\delme.dat", FileMode.Open, FileAccess.Read))
        {
            byte[] buffer = new byte[8];
            sr.Read(buffer, 0, 8);

            theDouble = BitConverter.ToDouble(buffer, 0);
        }

2) You can't.

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Here's how to read (and write for testing purposes) a double:

    // Write Double
    FileStream FS = new FileStream(@"C:\Test.bin", FileMode.Create);
    BinaryWriter BW = new BinaryWriter(FS);

    double Data = 123.456;

    BW.Write(Data);
    BW.Close();

    // Read Double
    FS = new FileStream(@"C:\Test.bin", FileMode.Open);
    BinaryReader BR = new BinaryReader(FS);

    Data = BR.ReadDouble();
    BR.Close();

Getting it out of a large file depends on how the data is laid out in the file.

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using (FileStream filestream = new FileStream(filename, FileMode.Open))
using (BinaryReader reader = new BinaryReader(filestream))
{
    float x = reader.ReadSingle();
}