Allow ConfigBinder to bind arrays to Singular elements

When trying to bind a collection of type `T`, ConfigBinder will now try
to bind to the current object instead of children objects when the
provided configuration doesn't contain an array.

This change allows elements that are not arrays to be bound to arrays.
For example, a type string[] can now be bound to "key" along with
"key:0".

src/libraries/Microsoft.Extensions.Configuration.Binder/src/ConfigurationBinder.cs

@@ -362,7 +362,7 @@ private static object BindInstance(
                 return convertedValue;
             }
 
-            if (config != null && config.GetChildren().Any())
+            if (config != null && (config.GetChildren().Any() || configValue != null))
             {
                 // If we don't have an instance, try to create one
                 if (instance == null)
@@ -495,7 +495,17 @@ private static void BindCollection(
             Type itemType = collectionType.GenericTypeArguments[0];
             MethodInfo addMethod = collectionType.GetMethod("Add", DeclaredOnlyLookup);
 
-            foreach (IConfigurationSection section in config.GetChildren())
+            IEnumerable<IConfigurationSection> children;
+            if (config.GetChildren().Any(a => long.TryParse(a.Key, out _)))
+            {
+                children = config.GetChildren();
+            }
+            else
+            {
+                children = new[] { config as IConfigurationSection };
+            }
+
+            foreach (IConfigurationSection section in children)
             {
                 try
                 {
@@ -518,7 +528,16 @@ private static void BindCollection(
         [RequiresUnreferencedCode("Cannot statically analyze what the element type is of the Array so its members may be trimmed.")]
         private static Array BindArray(Array source, IConfiguration config, BinderOptions options)
         {
-            IConfigurationSection[] children = config.GetChildren().ToArray();
+            IConfigurationSection[] children;
+            if (config.GetChildren().Any(a => long.TryParse(a.Key, out _)))
+            {
+                children = config.GetChildren().ToArray();
+            }
+            else
+            {
+                children = new[] { config as IConfigurationSection };
+            }
+
             int arrayLength = source.Length;
             Type elementType = source.GetType().GetElementType();
             var newArray = Array.CreateInstance(elementType, arrayLength + children.Length);

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/ConfigurationBinderTests.cs

@@ -934,6 +934,34 @@ public void ExceptionWhenTryingToBindToByteArray()
                 exception.Message);
         }
 
+        [Fact]
+        public void CanBindSingleElementToCollection()
+        {
+            var dic = new Dictionary<string, string>
+            {
+                {"MyString", "hello world"},
+                {"Nested:Integer", "11"},
+            };
+
+            var configurationBuilder = new ConfigurationBuilder();
+            configurationBuilder.AddInMemoryCollection(dic);
+            var config = configurationBuilder.Build();
+
+            var stringArr = config.GetSection("MyString").Get<string[]>();
+            Assert.Equal("hello world", stringArr[0]);
+            Assert.Equal(1, stringArr.Length);
+
+            var stringAsStr = config.GetSection("MyString").Get<string>();
+            Assert.Equal("hello world", stringAsStr);
+
+            var nested = config.GetSection("Nested").Get<NestedOptions>();
+            Assert.Equal(11, nested.Integer);
+
+            var nestedAsArray = config.GetSection("Nested").Get<NestedOptions[]>();
+            Assert.Equal(11, nestedAsArray[0].Integer);
+            Assert.Equal(1, nestedAsArray.Length);
+        }
+
         private interface ISomeInterface
         {
         }