Reinterpret 64-bit signed integer to a double-precision floating point number in C#

The BitConverter.Int64BitsToDouble() method in C# reinterprets the bit pattern of a 64-bit signed integer as a double-precision floating point number. This is not a numeric conversion − it treats the integer's binary representation as if it were the IEEE 754 double format.

Syntax

Following is the syntax for the Int64BitsToDouble method −

public static double Int64BitsToDouble(long value)

Parameters

• value − A 64-bit signed integer whose bit pattern will be reinterpreted as a double.

Return Value

Returns a double-precision floating point number whose bit representation is equivalent to the input long value.

Using Int64BitsToDouble with Small Values

Example

using System;
public class Demo {
   public static void Main() {
      long d = 20;
      Console.WriteLine("Value (64-bit signed integer) = " + d);
      double res = BitConverter.Int64BitsToDouble(d);
      Console.WriteLine("Value (double-precision floating point number) = " + res);
      Console.WriteLine("Scientific notation: " + res.ToString("E"));
   }
}

The output of the above code is −

Value (64-bit signed integer) = 20
Value (double-precision floating point number) = 9.88131291682493E-323
Scientific notation: 9.881313E-323

Using Int64BitsToDouble with Larger Values

Example

using System;
public class Demo {
   public static void Main() {
      long d = 9846587687;
      Console.WriteLine("Value (64-bit signed integer) = " + d);
      double res = BitConverter.Int64BitsToDouble(d);
      Console.WriteLine("Value (double-precision floating point number) = " + res);
      
      // Show hex representation for clarity
      Console.WriteLine("Hex representation of long: 0x" + d.ToString("X"));
      Console.WriteLine("Hex representation of double bits: 0x" + 
                       BitConverter.DoubleToInt64Bits(res).ToString("X"));
   }
}

The output of the above code is −

Value (64-bit signed integer) = 9846587687
Value (double-precision floating point number) = 4.86486070491012E-314
Hex representation of long: 0x24B1F3927
Hex representation of double bits: 0x24B1F3927

Round-trip Conversion Example

Example

using System;
public class Demo {
   public static void Main() {
      double original = 3.14159;
      Console.WriteLine("Original double: " + original);
      
      // Convert double to long bits
      long bits = BitConverter.DoubleToInt64Bits(original);
      Console.WriteLine("As long bits: " + bits);
      
      // Convert back to double
      double restored = BitConverter.Int64BitsToDouble(bits);
      Console.WriteLine("Restored double: " + restored);
      Console.WriteLine("Values equal: " + (original == restored));
   }
}

The output of the above code is −

Original double: 3.14159
As long bits: 4614256447914709615
Restored double: 3.14159
Values equal: True

Common Use Cases

• Binary serialization − Converting floating point values to integer format for storage or transmission.

• Low-level data manipulation − Working with the actual bit representation of floating point numbers.

• Reverse engineering − Analyzing binary data that contains encoded double values.

• Round-trip operations − Used with DoubleToInt64Bits for preserving exact bit patterns.

Conclusion

The BitConverter.Int64BitsToDouble() method performs bit-level reinterpretation, not numeric conversion. It treats the 64-bit integer's binary pattern as IEEE 754 double format, making it useful for low-level data operations and binary serialization scenarios.