Don't throw NRE when custom formatter returns null

2 files changed, 33 insertions, 10 deletions

diff --git a/mcs/class/corlib/System/String.cs b/mcs/class/corlib/System/String.cs
index 1ab450e1c47..9b345f17d72 100644
--- a/mcs/class/corlib/System/String.cs
+++ b/mcs/class/corlib/System/String.cs
@@ -1953,6 +1953,8 @@ namespace System
 
 			int ptr = 0;
 			int start = ptr;
+			var formatter = provider != null ? provider.GetFormat (typeof (ICustomFormatter)) as ICustomFormatter : null;
+
 			while (ptr < format.length) {
 				char c = format[ptr ++];
 
@@ -1981,21 +1983,21 @@ namespace System
 					object arg = args[n];
 
 					string str;
-					ICustomFormatter formatter = null;
-					if (provider != null)
-						formatter = provider.GetFormat (typeof (ICustomFormatter))
-							as ICustomFormatter;
 					if (arg == null)
 						str = Empty;
 					else if (formatter != null)
 						str = formatter.Format (arg_format, arg, provider);
-					else if (arg is IFormattable)
-						str = ((IFormattable)arg).ToString (arg_format, provider);
 					else
-						str = arg.ToString ();
+						str = null;
 
-					// pad formatted string and append to result
+					if (str == null) {
+						if (arg is IFormattable)
+							str = ((IFormattable)arg).ToString (arg_format, provider);
+						else
+							str = arg.ToString ();
+					}
 
+					// pad formatted string and append to result
 					if (width > str.length) {
 						const char padchar = ' ';
 						int padlen = width - str.length;
@@ -2008,9 +2010,9 @@ namespace System
 							result.Append (padchar, padlen);
 							result.Append (str);
 						}
-					}
-					else
+					} else {
 						result.Append (str);
+					}
 
 					start = ptr;
 				}
diff --git a/mcs/class/corlib/Test/System/StringTest.cs b/mcs/class/corlib/Test/System/StringTest.cs
index 429ff654f0a..ef92166c281 100644
--- a/mcs/class/corlib/Test/System/StringTest.cs
+++ b/mcs/class/corlib/Test/System/StringTest.cs
@@ -24,6 +24,20 @@ namespace MonoTests.System
 [TestFixture]
 public class StringTest
 {
+	class NullFormatter : IFormatProvider, ICustomFormatter
+	{
+		public string Format (string format, object arg, IFormatProvider provider)
+		{
+			return null;
+		}
+
+		public object GetFormat (Type formatType)
+		{
+			return this;
+		}
+	}
+
+
 	private CultureInfo orgCulture;
 
 	[SetUp]
@@ -1162,6 +1176,13 @@ public class StringTest
 	}
 
 	[Test]
+	public void Format ()
+	{
+		var s = String.Format (new NullFormatter (), "{0:}", "test");
+		Assert.AreEqual ("test", s);
+	}
+
+	[Test]
 	public void TestGetEnumerator ()
 	{
 		string s1 = "original";