feat: add GetSelectedCountries method to CountryProvider class

src/World.Net.Console/Program.cs

@@ -21,6 +21,24 @@
     Console.WriteLine($"Error: {ex.Message}");
 }
 
+PrintSeparator("Selected Countries");
+var indentifiers = new List<CountryIdentifier>
+{
+    CountryIdentifier.Afghanistan,
+    CountryIdentifier.Armenia,
+    CountryIdentifier.Niger,
+    CountryIdentifier.Nigeria,
+    CountryIdentifier.Brazil
+};
+
+var selectedCountries = CountryProvider.GetCountries(indentifiers);
+foreach (var country in selectedCountries)
+{
+    Console.WriteLine($"Name: {country.Name,-20} | Capital: {country.Capital}");
+}
+
+Console.WriteLine();
+
 Console.WriteLine();
 Console.WriteLine("Press any key to exit...");
 Console.ReadKey();

src/World.Net.UnitTests/CountryProviderTest.cs

@@ -1,6 +1,4 @@
-using World.Net.Helpers;
-
-namespace World.Net.UnitTests;
+namespace World.Net.UnitTests;
 
 public sealed class CountryProviderTest
 {
@@ -57,4 +55,59 @@ public void GetCountry_ShouldThrow_CountryNotFoundException_WhenCountryDoesNotEx
         var exception = Assert.Throws<CountryNotFoundException>(() => CountryProvider.GetCountry(nonExistingCountryId));
         Assert.Equal($"Country with id {nonExistingCountryId} was not found.", exception.Message);
     }
+
+    [Fact]
+    public void GetCountries_ValidIdentifiers_ReturnsCorrectCountries()
+    {
+        // Arrange
+        var indentifiers = new List<CountryIdentifier>
+        {
+            CountryIdentifier.Afghanistan,
+            CountryIdentifier.Brazil
+        };
+
+        // Act
+        var result = CountryProvider.GetCountries(indentifiers);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.NotEmpty(result);
+        Assert.Equal(indentifiers.Count, result.Count);
+    }
+
+    [Fact]
+    public void GetCountries_EmptyIdentifiers_ReturnsEmptyList()
+    {
+        // Arrange
+        var emptyIdentifiers = new List<CountryIdentifier>();
+
+        // Act
+        var result = CountryProvider.GetCountries(emptyIdentifiers);
+
+        // Assert
+        Assert.Empty(result);
+        Assert.Equal(emptyIdentifiers.Count, result.Count);
+    }
+
+    [Fact]
+    public void GetCountries_WhenIdentifiersContainDuplicates_ReturnsDistinctCountries()
+    {
+        // Arrange
+        var indentifiers = new List<CountryIdentifier>
+        {
+            CountryIdentifier.Afghanistan,
+            CountryIdentifier.Afghanistan,
+            CountryIdentifier.Argentina,
+            CountryIdentifier.Brazil,
+            CountryIdentifier.Brazil
+        };
+
+        // Act
+        var result = CountryProvider.GetCountries(indentifiers);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.NotEmpty(result);
+        Assert.Equal(3, result.Count);
+    }
 }

src/World.Net/CountryProvider.cs

@@ -55,4 +55,18 @@ public static ICountry GetCountry(CountryIdentifier countryIdentifier)
 
         throw new CountryNotFoundException($"Country with id {countryIdentifier} was not found.");
     }
+
+    /// <summary>
+    /// Retrieves multiple countries by their unique identifiers.
+    /// </summary>
+    /// <param name="countryIdentifiers">A collection of unique identifiers of the countries to retrieve.</param>
+    /// <returns>
+    /// A collection of <see cref="ICountry"/> instances corresponding to the specified <paramref name="countryIdentifiers"/>.
+    /// </returns>
+    public static IList<ICountry> GetCountries(IEnumerable<CountryIdentifier> countryIdentifiers)
+    {
+        var countries = _countries.Value.Where(x => countryIdentifiers.Contains(x.Key)).Select(x => x.Value);
+
+        return countries.ToList();
+    }
 }