Prevent string resource lookup if not required (#265)

Signed-off-by: Campbell Harding-Deason <campbell.harding-deason@firstaml.com>

Author: campbellhardingdeason

src/CloudNative.CloudEvents/CloudEvent.cs

@@ -122,7 +122,7 @@ public object? this[CloudEventAttribute attribute]
             get
             {
                 Validation.CheckNotNull(attribute, nameof(attribute));
-                Validation.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), () => Strings.ErrorCannotIndexBySpecVersionAttribute);
 
                 // TODO: Is this validation definitely useful? It does mean we never return something
                 // that's invalid for the attribute, which is potentially good...
@@ -136,7 +136,7 @@ public object? this[CloudEventAttribute attribute]
             set
             {
                 Validation.CheckNotNull(attribute, nameof(attribute));
-                Validation.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attribute.Name != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attribute), () => Strings.ErrorCannotIndexBySpecVersionAttribute);
 
                 string name = attribute.Name;
                 var knownAttribute = GetAttribute(name);
@@ -181,13 +181,13 @@ public object? this[string attributeName]
             {
                 // TODO: Validate the attribute name is valid (e.g. not upper case)? Seems overkill.
                 Validation.CheckNotNull(attributeName, nameof(attributeName));
-                Validation.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), () => Strings.ErrorCannotIndexBySpecVersionAttribute);
                 return attributeValues.GetValueOrDefault(Validation.CheckNotNull(attributeName, nameof(attributeName)));
             }            
             set
             {
                 Validation.CheckNotNull(attributeName, nameof(attributeName));
-                Validation.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), Strings.ErrorCannotIndexBySpecVersionAttribute);
+                Validation.CheckArgument(attributeName != CloudEventsSpecVersion.SpecVersionAttributeName, nameof(attributeName), () => Strings.ErrorCannotIndexBySpecVersionAttribute);
 
                 var knownAttribute = GetAttribute(attributeName);
 

src/CloudNative.CloudEvents/Core/Validation.cs

@@ -40,6 +40,20 @@ public static void CheckArgument([DoesNotReturnIf(false)] bool condition, string
             }
         }
 
+        /// <summary>
+        /// Validates an argument-dependent condition, throwing an exception if the check fails.
+        /// </summary>
+        /// <param name="condition">The condition to validate; this method will throw an <see cref="ArgumentException"/> if this is false.</param>
+        /// <param name="paramName">The name of the parameter being validated. May be null.</param>
+        /// <param name="messageFunc">A func that returns the message to use in the exception, if one is thrown.</param>
+        internal static void CheckArgument([DoesNotReturnIf(false)] bool condition, string paramName, Func<string> messageFunc)
+        {
+            if (!condition)
+            {
+                throw new ArgumentException(messageFunc(), paramName);
+            }
+        }
+
         /// <summary>
         /// Validates an argument-dependent condition, throwing an exception if the check fails.
         /// </summary>