Truncate (sanitize) decimal numbers to the Nth decimal place.

Sanitize.cs

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// File "CryptoExtentions.cs" last formatted on 2022-01-16 at 5:31 PM by Protiguous.

using System.Diagnostics.Contracts;

public class ProgramCrypto {

	public static void Main() {
		var random = new Random();

		for ( var i = 0; i < 100; i++ ) {
			var btc = ( Decimal )random.NextDouble() * 10000M;

			Console.WriteLine( btc.Sanitize().ToString( "G17" ) );
		}
	}

}

public static class CryptoExtentions {

	private const Byte CryptoDefaultDecimalPlacesHigh = CryptoDefaultDecimalPlaces * 2;

	/// <summary>The default for bitcoin (and similar crypto) truncates.</summary>
	private const Byte CryptoDefaultDecimalPlaces = 8;

	private static readonly Dictionary<Byte, (Decimal decrease, Decimal increase)> Tens = new();

	static CryptoExtentions() {
		for ( Byte i = 0; i < CryptoDefaultDecimalPlacesHigh; i++ ) {
			var decrease = ( Decimal )Math.Pow( 10, -CryptoDefaultDecimalPlaces );
			var increase = ( Decimal )Math.Pow( 10, CryptoDefaultDecimalPlaces );
			Tens[ i ] = (decrease, increase);
		}
	}

	/// <summary>Truncate, don't round. Just chop it off after <paramref name="decimalPlaces" />.</summary>
	/// <param name="number"></param>
	/// <param name="decimalPlaces"></param>
	[Pure]
	public static Decimal Sanitize( this Decimal number, Byte decimalPlaces = 8 ) {
		if ( Tens.TryGetValue( decimalPlaces, out var table ) ) {
			(var decrease, var increase) = table;
			number *= increase;
			number = Math.Truncate( number ); //Don't round. Just Truncate.
			number *= decrease;
		}
		else {
			number *= ( Decimal )Math.Pow( 10, decimalPlaces );
			number = Math.Truncate( number ); //Don't round. Just Truncate.
			number *= ( Decimal )Math.Pow( 10, -decimalPlaces );
		}

		return number;
	}

}

Added a precomputed lookup table for 8 to 16 decimal places for the powers of 10.