Fix bug in TextLoader throwing null exception in some conditions (#3056)

Author: yaeldMS

src/Microsoft.ML.Data/DataLoadSave/Text/TextLoader.cs

@@ -1288,12 +1288,9 @@ private static bool TryParseSchema(IHost host, IMultiStreamSource files,
                 ch.Assert(h.Loader == null || h.Loader is ICommandLineComponentFactory);
                 var loader = h.Loader as ICommandLineComponentFactory;
 
-                if (loader == null || string.IsNullOrWhiteSpace(loader.Name))
-                    goto LDone;
-
-                // Make sure the loader binds to us.
-                var info = host.ComponentCatalog.GetLoadableClassInfo<SignatureDataLoader>(loader.Name);
-                if (info.Type != typeof(ILegacyDataLoader) || info.ArgType != typeof(Options))
+                // Make sure that the schema is described using either the syntax TextLoader{<settings>} or the syntax Text{<settings>},
+                // where "settings" is a string that can be parsed by CmdParser into an object of type TextLoader.Options.
+                if (loader == null || string.IsNullOrWhiteSpace(loader.Name) || (loader.Name != LoaderSignature && loader.Name != "Text"))
                     goto LDone;
 
                 var optionsNew = new Options();

test/Microsoft.ML.Tests/TextLoaderTests.cs

@@ -598,6 +598,19 @@ public void ThrowsExceptionWithPropertyName()
             catch (NullReferenceException) { };
         }
 
+        [Fact]
+        public void ParseSchemaFromTextFile()
+        {
+            var mlContext = new MLContext(seed: 1);
+            var fileName = GetDataPath(TestDatasets.adult.trainFilename);
+            var loader = mlContext.Data.CreateTextLoader(new TextLoader.Options(), new MultiFileSource(fileName));
+            var data = loader.Load(new MultiFileSource(fileName));
+            Assert.NotNull(data.Schema.GetColumnOrNull("Label"));
+            Assert.NotNull(data.Schema.GetColumnOrNull("Workclass"));
+            Assert.NotNull(data.Schema.GetColumnOrNull("Categories"));
+            Assert.NotNull(data.Schema.GetColumnOrNull("NumericFeatures"));
+        }
+
         public class QuoteInput
         {
             [LoadColumn(0)]