Add IsNullOrEmpty to string (#117)

nanoFramework.CoreLibrary/System/AssemblyInfo.cs

@@ -14,4 +14,4 @@
 [assembly: AssemblyProduct("nanoFramework mscorlib")]
 [assembly: AssemblyCopyright("Copyright (c) .NET Foundation and Contributors")]
 
-[assembly: AssemblyNativeVersion("100.5.0.3")]
+[assembly: AssemblyNativeVersion("100.5.0.4")]

nanoFramework.CoreLibrary/System/String.cs

@@ -37,11 +37,11 @@ public IEnumerator GetEnumerator()
             throw new NotSupportedException(); 
         }
 
-
         /// <summary>
         /// Represents the empty string. This field is read-only.
         /// </summary>
         public static readonly String Empty = "";
+
         /// <summary>
         /// Determines whether this instance and a specified object, which must also be a String object, have the same value.
         /// </summary>
@@ -80,9 +80,6 @@ public override bool Equals(object obj)
         [MethodImpl(MethodImplOptions.InternalCall)]
         public static extern bool operator !=(String a, String b);
 
-
-
-
         /// <summary>
         /// Gets the Char object at a specified position in the current String object.
         /// </summary>
@@ -814,5 +811,15 @@ public String PadRight(int totalWidth, char paddingChar = ' ')
                 return this + new String(paddingChar, totalWidth - Length);
             }
         }
+
+        /// <summary>
+        /// Indicates whether the specified string is <see langword="null"/> or an empty string ("").
+        /// </summary>
+        /// <param name="value">The string to test.</param>
+        /// <returns><see langword="true"/> if the value parameter is <see langword="null"/> or an empty string (""); otherwise, <see langword="false"/>.</returns>
+        public static bool IsNullOrEmpty(string value)
+        {
+            return value == null || value.Length == 0;
+        }
     }
 }