Validate parsed enums and update tests

Refactor TryGetCountry to a shared internal implementation and add enum validation: only return a country when Enum.TryParse AND Enum.IsDefined for Alpha2Code/Alpha3Code. Update unit tests to use TestMethod, add a negative DataRow ("0", false), and strengthen assertions/messages. Also set an explicit numeric value for Alpha2Code.ZW = 9087.

- #61

Author: tinohager

src/Nager.Country.UnitTest/CountryTest.cs

@@ -8,7 +8,7 @@ namespace Nager.Country.UnitTest
     [TestClass]
     public class CountryTest
     {
-        [DataTestMethod]
+        [TestMethod]
         [DataRow("DE")]
         [DataRow("de")]
         [DataRow("De")]
@@ -26,7 +26,8 @@ public void GetCountry(string countryCode)
             }
         }
 
-        [DataTestMethod]
+        [TestMethod]
+        [DataRow("0", false)]
         [DataRow("DE", true)]
         [DataRow("de", true)]
         [DataRow("De", true)]
@@ -41,17 +42,19 @@ public void TryGetCountry(string countryCode, bool isSuccessful)
             var successful = countryProvider.TryGetCountry(countryCode, out var countryInfo);
             if (isSuccessful)
             {
-                Assert.IsTrue(successful);
+                Assert.IsTrue(successful, "The assignment should be possible.");
                 Assert.IsNotNull(countryInfo);
+
+                Assert.AreEqual(Alpha2Code.DE, countryInfo.Alpha2Code);
             }
             else
             {
-                Assert.IsFalse(successful);
+                Assert.IsFalse(successful, "The assignment should not be possible.");
                 Assert.IsNull(countryInfo);
             }
         }
 
-        [DataTestMethod]
+        [TestMethod]
         [DataRow("Austria")]
         [DataRow("Republic of Austria")]
         public void GetCountryByName(string countryName)
@@ -62,7 +65,7 @@ public void GetCountryByName(string countryName)
             Assert.AreEqual(Alpha2Code.AT, countryInfo.Alpha2Code);
         }
 
-        [DataTestMethod]
+        [TestMethod]
         [DataRow("Austria")]
         [DataRow("Republic of Austria")]
         [DataRow("austria")]

src/Nager.Country/Alpha2Code.cs

@@ -1008,6 +1008,6 @@ public enum Alpha2Code : int
         /// <summary>
         /// Zimbabwe
         /// </summary>
-        ZW
+        ZW = 9087
     }
 }

src/Nager.Country/CountryProvider.cs

@@ -313,20 +313,7 @@ public ICountryInfo GetCountry(string alpha2or3Code)
         /// <inheritdoc/>
         public bool TryGetCountry(string alpha2or3Code, out ICountryInfo? countryInfo)
         {
-            if (Enum.TryParse(alpha2or3Code, true, out Alpha2Code alpha2Code))
-            {
-                countryInfo = this.GetCountry(alpha2Code);
-                return true;
-            }
-
-            if (Enum.TryParse(alpha2or3Code, true, out Alpha3Code alpha3Code))
-            {
-                countryInfo = this.GetCountry(alpha3Code);
-                return true;
-            }
-
-            countryInfo = null;
-            return false;
+            return this.TryGetCountryInternal(alpha2or3Code, out countryInfo);
         }
 
 #endif
@@ -335,25 +322,36 @@ public bool TryGetCountry(string alpha2or3Code, out ICountryInfo? countryInfo)
 
         /// <inheritdoc/>
         public bool TryGetCountry(string alpha2or3Code, [NotNullWhen(true)] out ICountryInfo? countryInfo)
+        {
+            return this.TryGetCountryInternal(alpha2or3Code, out countryInfo);
+        }
+
+#endif
+
+        private bool TryGetCountryInternal(string alpha2or3Code, out ICountryInfo? countryInfo)
         {
             if (Enum.TryParse(alpha2or3Code, true, out Alpha2Code alpha2Code))
             {
-                countryInfo = this.GetCountry(alpha2Code);
-                return true;
+                if (Enum.IsDefined(typeof(Alpha2Code), alpha2Code))
+                {
+                    countryInfo = this.GetCountry(alpha2Code);
+                    return true;
+                }
             }
 
             if (Enum.TryParse(alpha2or3Code, true, out Alpha3Code alpha3Code))
             {
-                countryInfo = this.GetCountry(alpha3Code);
-                return true;
+                if (Enum.IsDefined(typeof(Alpha3Code), alpha3Code))
+                {
+                    countryInfo = this.GetCountry(alpha3Code);
+                    return true;
+                }
             }
 
             countryInfo = null;
             return false;
         }
 
-#endif
-
         /// <inheritdoc/>
         public ICountryInfo GetCountry(Alpha2Code alpha2Code)
         {