check for number of input columns in concat transform (#3809)

Author: abgoswam

src/Microsoft.ML.Data/Transforms/ColumnConcatenatingEstimator.cs

@@ -54,6 +54,7 @@ internal ColumnConcatenatingEstimator(IHostEnvironment env, string outputColumnN
 
             _host.CheckNonEmpty(outputColumnName, nameof(outputColumnName));
             _host.CheckValue(inputColumnNames, nameof(inputColumnNames));
+            _host.CheckParam(inputColumnNames.Length > 0, nameof(inputColumnNames), "Input columns not specified");
             _host.CheckParam(!inputColumnNames.Any(r => string.IsNullOrEmpty(r)), nameof(inputColumnNames),
                 "Contained some null or empty items");
 

test/Microsoft.ML.Tests/Transformers/ConcatTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.IO;
 using Microsoft.ML.Data;
 using Microsoft.ML.Data.IO;
@@ -17,6 +18,25 @@ public ConcatTests(ITestOutputHelper output) : base(output)
         {
         }
 
+        [Fact]
+        void TestConcatNoInputColumns()
+        {
+            var thrown = false;
+
+            try
+            {
+                var pipe = ML.Transforms.Concatenate("Features");
+            }
+            catch(Exception ex)
+            {
+                Assert.Contains("Input columns not specified", ex.Message);
+                thrown = true;
+
+            }
+            Assert.True(thrown);
+            Done();
+        }
+
         [Fact]
         void TestConcat()
         {