Add day 4

StringExtensions.cs

@@ -1,6 +1,6 @@
 namespace AdventOfCode;
 
-public static partial class StringExtensions
+public static class StringExtensions
 {
     public static IEnumerable<string> ReadAllLines(this StringReader reader)
     {
@@ -14,4 +14,25 @@ public static partial class StringExtensions
     {
         return new StringReader(text).ReadAllLines();
     }
+
+    public static char[,] AsCharGrid(this string text)
+    {
+        var lines = text.ReadAllLines().ToArray();
+        var lineCount = lines.Length;
+        var columnCount = lines[0].Length;
+
+        var grid = new char[lineCount, columnCount];
+
+        for (int i = 0; i < lineCount; i++)
+        {
+            var line = lines[i];
+
+            for (int j = 0; j < columnCount; j++)
+            {
+                grid[i, j] = line[j];
+            }
+        }
+
+        return grid;
+    }
 }