Fix null parms in String.Format (#144)

Author: edleno2

Tests/NFUnitTestSystemLib/UnitTestStringTests.cs

@@ -315,6 +315,28 @@ public void Concat_Test1()
             string str = "a" + 1 + "b" + new ToStringReturnsNull();
             Assert.Equal(str, "a1b");
         }
+
+        [TestMethod]
+        public void Format()
+        {
+            Assert.Equal(String.Format("The value is {0}", 32), "The value is 32",
+                "String.Format with a single variable");
+            Assert.Equal(String.Format("The value with formatter is {0:d3}", 32), "The value with formatter is 032",
+                "String.Format with a decimal formatter.");
+        }
+
+        [TestMethod]
+        public void FormatWithNull()
+        {
+            object nullObject = null;
+            Assert.Equal(String.Format("The value is {0}", nullObject), "The value is ", 
+                "String.Format should treat a null argument as an empty string");
+            Assert.Equal(String.Format("The value with formatter is {0:d}", nullObject), "The value with formatter is ", 
+                "String.Format should treat a null argument as an empty string when used with formatters.");
+            Assert.Equal(String.Format("First parm: '{0}' second parm: '{1}'", new object[] { nullObject, 32 }), "First parm: '' second parm: '32'", 
+                "Formatting with two parms should also work");
+            Assert.Equal($"the value is {nullObject}", "the value is ", "Interpolated strings should use string.format and work correctly");
+        }
     }
 
     /// <summary>

nanoFramework.CoreLibrary/System/String.cs

@@ -709,18 +709,26 @@ public static string Format(string format, params object[] args)
                     if (fmt.Length > 0)
                     {
 #if NANOCLR_REFLECTION
-                        var method = args[index].GetType().GetMethod("ToString", new Type[] { typeof(string) });
-                        token = (method is null)
-                            ? args[index].ToString()
-                            : method.Invoke(args[index], new object[] { token }).ToString();
+                        
+                        if (args[index] is null)
+                        {
+                           token = "";
+                        }
+                        else
+                        {
+                            var method = args[index].GetType().GetMethod("ToString", new Type[] { typeof(string) });
+                            token = (method is null)
+                                ? args[index].ToString()
+                                : method.Invoke(args[index], new object[] { token }).ToString();
+                        }
 #else
                         throw new NotImplementedException();
 #endif // NANOCLR_REFLECTION
 
                     }
                     else
                     {
-                        token = args[index].ToString();
+                        token = args[index] == null ? "" : args[index].ToString();
                     }
 
                     if (alignment > 0)