Pay button was not working properly if the server was not en-US (Fix #638)

BTCPayServer/ModelBinders/InvariantDecimalModelBinder.cs

@@ -0,0 +1,99 @@
+// Copied and adjusted from https://github.com/aspnet/Mvc/blob/master/src/Microsoft.AspNetCore.Mvc.Core/ModelBinding/Binders/DecimalModelBinder.cs
+using System;
+using System.Globalization;
+using System.Runtime.ExceptionServices;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+
+namespace BTCPayServer.ModelBinders
+{
+    /// <summary>
+    /// An <see cref="IModelBinder"/> for <see cref="decimal"/> and <see cref="Nullable{T}"/> where <c>T</c> is
+    /// <see cref="decimal"/>.
+    /// </summary>
+    public class InvariantDecimalModelBinder : IModelBinder
+    {
+        private readonly NumberStyles _supportedStyles;
+
+        public InvariantDecimalModelBinder()
+        {
+            _supportedStyles = NumberStyles.Any;
+        }
+
+        /// <inheritdoc />
+        public Task BindModelAsync(ModelBindingContext bindingContext)
+        {
+            if (bindingContext == null)
+            {
+                throw new ArgumentNullException(nameof(bindingContext));
+            }
+
+            var modelName = bindingContext.ModelName;
+            var valueProviderResult = bindingContext.ValueProvider.GetValue(modelName);
+            if (valueProviderResult == ValueProviderResult.None)
+            {
+                return Task.CompletedTask;
+            }
+
+            var modelState = bindingContext.ModelState;
+            modelState.SetModelValue(modelName, valueProviderResult);
+
+            var metadata = bindingContext.ModelMetadata;
+            var type = metadata.UnderlyingOrModelType;
+            try
+            {
+                var value = valueProviderResult.FirstValue;
+                var culture = CultureInfo.InvariantCulture;
+
+                object model;
+                if (string.IsNullOrWhiteSpace(value))
+                {
+                    // Parse() method trims the value (with common NumberStyles) then throws if the result is empty.
+                    model = null;
+                }
+                else if (type == typeof(decimal))
+                {
+                    model = decimal.Parse(value, _supportedStyles, culture);
+                }
+                else
+                {
+                    // unreachable
+                    throw new NotSupportedException();
+                }
+
+                // When converting value, a null model may indicate a failed conversion for an otherwise required
+                // model (can't set a ValueType to null). This detects if a null model value is acceptable given the
+                // current bindingContext. If not, an error is logged.
+                if (model == null && !metadata.IsReferenceOrNullableType)
+                {
+                    modelState.TryAddModelError(
+                        modelName,
+                        metadata.ModelBindingMessageProvider.ValueMustNotBeNullAccessor(
+                            valueProviderResult.ToString()));
+                }
+                else
+                {
+                    bindingContext.Result = ModelBindingResult.Success(model);
+                }
+            }
+            catch (Exception exception)
+            {
+                var isFormatException = exception is FormatException;
+                if (!isFormatException && exception.InnerException != null)
+                {
+                    // Unlike TypeConverters, floating point types do not seem to wrap FormatExceptions. Preserve
+                    // this code in case a cursory review of the CoreFx code missed something.
+                    exception = ExceptionDispatchInfo.Capture(exception.InnerException).SourceException;
+                }
+
+                modelState.TryAddModelError(modelName, exception, metadata);
+
+                // Conversion failed.
+            }
+
+            return Task.CompletedTask;
+        }
+    }
+}

BTCPayServer/Models/StoreViewModels/PayButtonViewModel.cs

@@ -3,11 +3,14 @@ using System.Collections.Generic;
 using System.ComponentModel.DataAnnotations;
 using System.Linq;
 using System.Threading.Tasks;
+using BTCPayServer.ModelBinders;
+using Microsoft.AspNetCore.Mvc;
 
 namespace BTCPayServer.Models.StoreViewModels
 {
     public class PayButtonViewModel
     {
+        [ModelBinder(BinderType = typeof(InvariantDecimalModelBinder))]
         public decimal Price { get; set; }
         public string InvoiceId { get; set; }
         [Required]