Blazor: Validate Credit Card Numbers Using Luhn Check

This example is using .NET Core 3.1 and is compatible with .NET 5, .NET 6 and .NET 8.

The following code will use the Luhn check to validate a credit card on the client-side. It does not validate on the server-side which should be done. The point of this is to catch user-entry errors. The code does allow the user to use a number that fails the Luhn check.

Download the credit card icons: BLAZORCREDITCARD.zip.

Create or Modify a RAZOR Page

Create or modify a .razor page. In this case, Index.razor was modified.

@page "/"
@using System.Text.RegularExpressions

<h1>@(heading.Length == 0 ? "Pay Invoice" : heading)</h1>

<form style="max-width:400px;" class="@(heading.Length == 0 ? "" : "d-none")" onsubmit="return false;">
	<div class="mb-3">
		<input type="text" class="form-control" style="color:@(ValidateCreditCardNumber() ? "#0a0" : (cardNumber.Length == 0 ? "#000" : "red"));"
			   placeholder="Credit card number" @bind-value="cardNumber" @onblur="Blur" />
	</div>
	<div class="mb-3">
		<input type="text" class="form-control" placeholder="Security code" @bind-value="securityCode" />
	</div>
	<button type="submit" class="btn btn-primary" @onclick="Submit">Submit</button>&nbsp;&nbsp;<img src="@cc[ccid]" />
</form>

<div class="modal-backdrop fade @(show ? "show" : "") @(display ? "" : "d-none")"></div>
<div class="modal fade @(show ? "show" : "")  @(display ? "d-block" : "d-none")" tabindex="-1">
	<div class="modal-dialog">
		<div class="modal-content">
			<div class="modal-header">
				<h5 class="modal-title"><img src="@cc[ccid]" /></h5>
				<button type="button" class="btn-close" @onclick="Close">&times;</button>
			</div>
			<div class="modal-body">
				<p>@modalBodyText</p>
			</div>
			<div class="modal-footer">
				<button type="button" class="btn btn-primary @(modalButtonText.Length == 0 ? "d-none" : "")" @onclick="Confirm">@modalButtonText</button>
			</div>
		</div>
	</div>
</div>

@code {
	string heading = "";
	string cardNumber = "", securityCode = "";
	int ccid = 0;
	string[] cc = { "/img/blank.png", "/img/blank.png", "/img/blank.png", "/img/amex.png", "/img/visa.png", "/img/mastercard.png", "/img/discover.png", "/img/blank.png", "/img/blank.png", "/img/blank.png" };
	bool show, display;
	string modalButtonText = "", modalBodyText = "";

	bool ValidateCreditCardNumber() // The Luhn Check
	{
		var ccnum = cardNumber.Replace(" ", "");
		if (string.IsNullOrWhiteSpace(ccnum) || ccnum.Length < 15 || ccnum.Length > 16)
			return false;
		var rev = ccnum.Reverse(); // Luhn check approaches the card number in reversed order
		int sum = 0, i = 0;
		foreach (char c in rev)
		{
			if (c < '0' || c > '9')
				return false;
			int tmp = c - '0';
			if ((i & 1) != 0)
			{
				tmp <<= 1;
				if (tmp > 9)
					tmp -= 9;
			}
			sum += tmp;
			i++;
		}
		return ((sum % 10) == 0);
	}

	async Task Submit()
	{
		if (cardNumber.Length == 0 || securityCode.Length == 0)
		{
			modalButtonText = "";
			modalBodyText = "Both the credit card number and security code are required.";
			display = true;
			await Task.Delay(100);
			show = true;
			return;
		}
		if (!ValidateCreditCardNumber())
		{
			modalButtonText = "Use this Card Number";
			modalBodyText = "The credit card number " + cardNumber + " could not be verified. Are you sure you want to use it?";
			display = true;
			await Task.Delay(100);
			show = true;
			return;
		}
		heading = "Invoice Paid. Thank you.";
	}

	void Blur()
	{
		cardNumber = Regex.Replace(cardNumber.Trim(), "[^0-9 ]", "");
		ccid = (cardNumber.Length == 0) ? 0 : (cardNumber[0] - '0');
	}

	async Task Close()
	{
		show = false;
		await Task.Delay(500);
		display = false;
	}

	async Task Confirm()
	{
		heading = "Processing payment... Thank you.";
		await Close();
	}
}

Coding Video

https://youtu.be/H8JmlhxJr60

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