Added RemoveAt extension method to StringBuilder

Kotz.Extensions/README.md

@@ -79,7 +79,8 @@ Defines the following extension methods:
     - ToTitleCase: Converts the current string to the *Title Case* format.
 - **StringBuilder Extensions**
     - ReplaceAll: Replaces all instances of a substring with another substring, even if the new substring is a substring of the old substring.
-    - Remove: Removes the specified string chunks from the current StringBuilder.
+    - Remove: Removes the specified string chunks from the current string builder.
+    - RemoveAt: Removes the character at the specified index.
     - ToStringAndClear: Returns the `string` value of the current builder and clears the builder.
     - Trim: Removes all leading and trailing instances of a character from the current string builder.
     - TrimEnd: Removes all trailing instances of a character from the current string builder.

Kotz.Extensions/StringBuilderExt.cs

@@ -113,8 +113,19 @@ public static StringBuilder Remove(this StringBuilder stringBuilder, IEnumerable
         return stringBuilder;
     }
 
-    // TODO: unit tests for the methods above
-    // TODO: RemoveAt(int)
+    /// <summary>
+    /// Removes the character at the specified <paramref name="index"/> from this string builder.
+    /// </summary>
+    /// <param name="stringBuilder">This string builder.</param>
+    /// <param name="index">The index of the character to be removed.</param>
+    /// <returns>A reference to this instance with the character removed.</returns>
+    /// <exception cref="ArgumentOutOfRangeException">Occurs when the index is less than zero or  greater than the length of the string builder.</exception>
+    public static StringBuilder RemoveAt(this StringBuilder stringBuilder, int index)
+    {
+        return (index >= stringBuilder.Length)
+            ? throw new ArgumentOutOfRangeException(nameof(index), index, "Index cannot be less than zero or greater than the length of the StringBuilder.")
+            : stringBuilder.Remove(index, 1);
+    }
 
     /// <summary>
     /// Converts the value of this instance to a <see langword="string"/>, then clears its buffer.

Kotz.Tests/Extensions/StringBuilderExtTests.cs

@@ -46,6 +46,7 @@ internal void ContainsTest(string firstChunk, string secondChunk, string toSearc
     [Theory]
     [InlineData("Hello World", "Hello World")]
     [InlineData("Hello World", "Hello World", "")]
+    [InlineData("Hello World", "HelloWorld", " ")]
     [InlineData("Hello World", "Hell Wrld", "o")]
     [InlineData("Hello World", "Hll Wrld", "o", "e")]
     [InlineData("Hello World", "Hello ", "World")]
@@ -61,6 +62,24 @@ internal void RemoveTest(string input, string expected, params string[] toRemove
     internal void RemoveExceptionTest()
         => Assert.Throws<ArgumentNullException>(() => new StringBuilder().Remove(null!));
 
+    [Theory]
+    [InlineData("Hello", "ello", 0)]
+    [InlineData("Hello", "Helo", 2)]
+    [InlineData("Hello", "Hell", 4)]
+    internal void RemoveAtTest(string input, string expected, int index)
+    {
+        var stringBuilder = new StringBuilder(input)
+            .RemoveAt(index);
+
+        Assert.Equal(expected, stringBuilder.ToString());
+    }
+
+    [Theory]
+    [InlineData("Hello", 5)]
+    [InlineData("Hello", -1)]
+    internal void RemoveAtException(string input, int index)
+        => Assert.Throws<ArgumentOutOfRangeException>(() => new StringBuilder(input).RemoveAt(index));
+
     [Theory]
     [InlineData("")]
     [InlineData("hello")]