C# Program to convert a Double value to an Int64 value

To convert a Double value to an Int64 value, use the Convert.ToInt64() method. This method performs a rounded conversion from a 64-bit floating-point number to a 64-bit signed integer.

Int64 represents a 64-bit signed integer and is equivalent to the long data type in C#. The conversion truncates the decimal portion and rounds to the nearest integer value.

Syntax

Following is the syntax for converting a double to Int64 −

long result = Convert.ToInt64(doubleValue);

Parameters

• doubleValue − A double-precision floating-point number to be converted.

Return Value

Returns a 64-bit signed integer that is equivalent to the rounded value of the double parameter.

Using Convert.ToInt64() with Scientific Notation

The method works seamlessly with scientific notation values. Let's convert a large double value expressed in scientific notation −

using System;

public class Demo {
   public static void Main() {
      double val = 23.951213e12;
      long longVal = Convert.ToInt64(val);
      Console.WriteLine("Original double: {0:E}", val);
      Console.WriteLine("Converted Int64: {0:N0}", longVal);
   }
}

The output of the above code is −

Original double: 2.395121E+012
Converted Int64: 23,951,213,000,000

Using Convert.ToInt64() with Decimal Values

When converting doubles with decimal portions, the method performs rounding to the nearest integer −

using System;

public class Demo {
   public static void Main() {
      double val1 = 123.45;
      double val2 = 123.67;
      double val3 = -456.23;
      
      long longVal1 = Convert.ToInt64(val1);
      long longVal2 = Convert.ToInt64(val2);
      long longVal3 = Convert.ToInt64(val3);
      
      Console.WriteLine("{0} converts to {1}", val1, longVal1);
      Console.WriteLine("{0} converts to {1}", val2, longVal2);
      Console.WriteLine("{0} converts to {1}", val3, longVal3);
   }
}

The output of the above code is −

123.45 converts to 123
123.67 converts to 124
-456.23 converts to -456

Using Convert.ToInt64() with Range Validation

It's important to handle cases where the double value might exceed the Int64 range to avoid overflow exceptions −

using System;

public class Demo {
   public static void Main() {
      double[] values = { 9223372036854775807.0, -9223372036854775808.0, 123.456 };
      
      foreach (double val in values) {
         try {
            long result = Convert.ToInt64(val);
            Console.WriteLine("Double {0} ? Int64 {1}", val, result);
         } catch (OverflowException) {
            Console.WriteLine("Double {0} is outside Int64 range", val);
         }
      }
   }
}

The output of the above code is −

Double 9.223372036854776E+18 ? Int64 9223372036854775807
Double -9.223372036854776E+18 ? Int64 -9223372036854775808
Double 123.456 ? Int64 123

Conclusion

The Convert.ToInt64() method provides a reliable way to convert double values to 64-bit signed integers with automatic rounding. Always consider potential overflow exceptions when working with very large double values that may exceed the Int64 range.