Fix TextLoader constructor and add exception message

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

@@ -1456,10 +1456,13 @@ internal static TextLoader CreateTextLoader<TInput>(IHostEnvironment host,
             var propertyInfos =
                 userType
                 .GetProperties(BindingFlags.Public | BindingFlags.Instance)
-                .Where(x => x.CanRead && x.CanWrite && x.GetGetMethod() != null && x.GetSetMethod() != null && x.GetIndexParameters().Length == 0);
+                .Where(x => x.CanRead && x.GetGetMethod() != null && x.GetIndexParameters().Length == 0);
 
             var memberInfos = (fieldInfos as IEnumerable<MemberInfo>).Concat(propertyInfos).ToArray();
 
+            if (memberInfos.Length == 0)
+                throw host.ExceptParam(nameof(TInput), $"Should define at least one public, readable field or property in {nameof(TInput)}.");
+
             var columns = new List<Column>();
 
             for (int index = 0; index < memberInfos.Length; index++)

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

@@ -62,10 +62,14 @@ public static TextLoader CreateTextLoader(this DataOperationsCatalog catalog,
         /// <summary>
         /// Create a text loader <see cref="TextLoader"/> by inferencing the dataset schema from a data model type.
         /// </summary>
+        /// <typeparam name="TInput">Defines the schema of the data to be loaded. Use public fields or properties
+        /// decorated with <see cref="LoadColumnAttribute"/> (and possibly other attributes) to specify the column
+        /// names and their data types in the schema of the loaded data.</typeparam>
         /// <param name="catalog">The <see cref="DataOperationsCatalog"/> catalog.</param>
         /// <param name="separatorChar">Column separator character. Default is '\t'</param>
         /// <param name="hasHeader">Does the file contains header?</param>
-        /// <param name="dataSample">The optional location of a data sample. The sample can be used to infer column names and number of slots in each column.</param>
+        /// <param name="dataSample">The optional location of a data sample. The sample can be used to infer information
+        /// about the columns, such as slot names.</param>
         /// <param name="allowQuoting">Whether the input may include quoted values,
         /// which can contain separator characters, colons,
         /// and distinguish empty values from missing values. When true, consecutive separators

test/Microsoft.ML.Tests/TextLoaderTests.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using Microsoft.ML.Data;
 using Microsoft.ML.Model;
 using Microsoft.ML.RunTests;
@@ -795,5 +796,97 @@ public void TestTextLoaderKeyTypeBackCompat()
                 Assert.True(result.Schema[featureIdx].Type is KeyDataViewType keyType && keyType.Count == typeof(uint).ToMaxInt());
             }
         }
+
+        private class IrisNoFields
+        {
+        }
+
+        private class IrisPrivateFields
+        {
+            [LoadColumn(0)]
+            private float SepalLength;
+
+            [LoadColumn(1)]
+            private float SepalWidth { get; }
+
+            public float GetSepalLenght()
+                => SepalLength;
+
+            public void SetSepalLength(float sepalLength)
+            {
+                SepalLength = sepalLength;
+            }
+        }
+        private class IrisPublicGetProperties
+        {
+            [LoadColumn(0)]
+            public float SepalLength { get; }
+
+            [LoadColumn(1)]
+            public float SepalWidth { get; }
+        }
+
+        private class IrisPublicFields
+        {
+            public IrisPublicFields(float sepalLength, float sepalWidth)
+            {
+                SepalLength = sepalLength;
+                SepalWidth = sepalWidth;
+            }
+
+            [LoadColumn(0)]
+            public readonly float SepalLength;
+
+            [LoadColumn(1)]
+            public float SepalWidth;
+        }
+
+        private class IrisPublicProperties
+        {
+            [LoadColumn(0)]
+            public float SepalLength { get; set; }
+
+            [LoadColumn(1)]
+            public float SepalWidth { get; set; }
+        }
+
+        [Fact]
+        public void TestTextLoaderNoFields()
+        {
+            var dataPath = GetDataPath(TestDatasets.irisData.trainFilename);
+            var mlContext = new MLContext();
+
+            // Class with get property only.
+            var dataIris = mlContext.Data.CreateTextLoader<IrisPublicGetProperties>(separatorChar: ',').Load(dataPath);
+            var oneIrisData = mlContext.Data.CreateEnumerable<IrisPublicProperties>(dataIris, false).First();
+            Assert.True(oneIrisData.SepalLength != 0 && oneIrisData.SepalWidth != 0);
+
+            // Class with read only fields.
+            dataIris = mlContext.Data.CreateTextLoader<IrisPublicFields>(separatorChar: ',').Load(dataPath);
+            oneIrisData = mlContext.Data.CreateEnumerable<IrisPublicProperties>(dataIris, false).First();
+            Assert.True(oneIrisData.SepalLength != 0 && oneIrisData.SepalWidth != 0);
+
+            // Class with no fields.
+            try
+            {
+                dataIris = mlContext.Data.CreateTextLoader<IrisNoFields>(separatorChar: ',').Load(dataPath);
+                Assert.False(true);
+            }
+            catch (Exception ex)
+            {
+                Assert.StartsWith("Should define at least one public, readable field or property in TInput.", ex.Message);
+            }
+
+            // Class with no public readable fields.
+            try
+            {
+                dataIris = mlContext.Data.CreateTextLoader<IrisPrivateFields>(separatorChar: ',').Load(dataPath);
+                Assert.False(true);
+            }
+            catch (Exception ex)
+            {
+                Assert.StartsWith("Should define at least one public, readable field or property in TInput.", ex.Message);
+            }
+        }
     }
 }