Convert Decimal to the equivalent 64-bit unsigned integer in C#

To convert the value of a Decimal to the equivalent 64-bit unsigned integer (ulong), C# provides the Decimal.ToUInt64() method. This method truncates the decimal portion and returns only the integer part as a ulong value.

Syntax

Following is the syntax for converting a decimal to 64-bit unsigned integer −

ulong result = Decimal.ToUInt64(decimalValue);

Parameters

• decimalValue − A Decimal number to be converted to a 64-bit unsigned integer.

Return Value

Returns a ulong value equivalent to the decimal value, with the fractional part truncated (not rounded).

Using Decimal.ToUInt64() with Fractional Values

When converting decimals with fractional parts, the method truncates (cuts off) the decimal portion −

using System;

public class Demo {
   public static void Main() {
      Decimal val = 99.29m;
      Console.WriteLine("Decimal value = " + val);
      ulong res = Decimal.ToUInt64(val);
      Console.WriteLine("64-bit unsigned integer = " + res);
   }
}

The output of the above code is −

Decimal value = 99.29
64-bit unsigned integer = 99

Using Decimal.ToUInt64() with Small Decimal Values

Values less than 1 will result in 0 when converted to unsigned integer −

using System;

public class Demo {
   public static void Main() {
      Decimal val = 0.001m;
      Console.WriteLine("Decimal value = " + val);
      ulong res = Decimal.ToUInt64(val);
      Console.WriteLine("64-bit unsigned integer = " + res);
   }
}

The output of the above code is −

Decimal value = 0.001
64-bit unsigned integer = 0

Using Decimal.ToUInt64() with Large Values

The method handles large decimal values within the ulong range −

using System;

public class Demo {
   public static void Main() {
      Decimal val1 = 18446744073709551615m; // Max ulong value
      Decimal val2 = 1234567890123.456m;
      
      Console.WriteLine("Decimal value 1 = " + val1);
      ulong res1 = Decimal.ToUInt64(val1);
      Console.WriteLine("64-bit unsigned integer 1 = " + res1);
      
      Console.WriteLine("\nDecimal value 2 = " + val2);
      ulong res2 = Decimal.ToUInt64(val2);
      Console.WriteLine("64-bit unsigned integer 2 = " + res2);
   }
}

The output of the above code is −

Decimal value 1 = 18446744073709551615
64-bit unsigned integer 1 = 18446744073709551615

Decimal value 2 = 1234567890123.456
64-bit unsigned integer 2 = 1234567890123

Key Rules

• The fractional part is truncated, not rounded.

• Values must be within the ulong range (0 to 18,446,744,073,709,551,615).

• Negative decimal values will throw an OverflowException.

• Values greater than ulong.MaxValue will throw an OverflowException.

Conclusion

The Decimal.ToUInt64() method converts decimal values to 64-bit unsigned integers by truncating the fractional part. This method is useful when you need to extract only the whole number portion of a decimal value as a ulong type.