Add support for String indexer property

Fixes #838

Author: hazzik

src/NHibernate.Test/Async/Linq/FunctionTests.cs

@@ -111,6 +111,19 @@ where e.FirstName.Substring(3) == "rew"
 			Assert.That(query[0].FirstName, Is.EqualTo("Andrew"));
 		}
 
+		[Test]
+		public async Task GetCharsFunctionAsync()
+		{
+			var query = await ((
+				from e in db.Employees
+				where e.FirstName[2] == 'e'
+				select e
+			).ToListAsync());
+
+			Assert.That(query.Count, Is.EqualTo(1));
+			Assert.That(query[0].FirstName, Is.EqualTo("Steven"));
+		}
+
 		[Test]
 		public async Task LeftFunctionAsync()
 		{

src/NHibernate.Test/Linq/FunctionTests.cs

@@ -100,6 +100,19 @@ where e.FirstName.Substring(3) == "rew"
 			Assert.That(query[0].FirstName, Is.EqualTo("Andrew"));
 		}
 
+		[Test]
+		public void GetCharsFunction()
+		{
+			var query = (
+				from e in db.Employees
+				where e.FirstName[2] == 'e'
+				select e
+			).ToList();
+
+			Assert.That(query.Count, Is.EqualTo(1));
+			Assert.That(query[0].FirstName, Is.EqualTo("Steven"));
+		}
+
 		[Test]
 		public void LeftFunction()
 		{

src/NHibernate/Linq/Functions/DefaultLinqToHqlGeneratorsRegistry.cs

@@ -41,6 +41,7 @@ public DefaultLinqToHqlGeneratorsRegistry()
 			this.Merge(new IndexOfGenerator());
 			this.Merge(new ReplaceGenerator());
 			this.Merge(new LengthGenerator());
+			this.Merge(new GetCharsGenerator());
 			this.Merge(new TrimGenerator());
 			this.Merge(new MathGenerator());
 

src/NHibernate/Linq/Functions/StringGenerator.cs

@@ -183,6 +183,24 @@ public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject,
 			return treeBuilder.MethodCall("substring", stringExpr, start, length);
 		}
 	}
+	
+	public class GetCharsGenerator : BaseHqlGeneratorForMethod
+	{
+		public GetCharsGenerator()
+		{
+			SupportedMethods = new[]
+			{
+				ReflectHelper.GetMethod<string, char>(s => s[0])
+			};
+		}
+
+		public override HqlTreeNode BuildHql(MethodInfo method, Expression targetObject, ReadOnlyCollection<Expression> arguments, HqlTreeBuilder treeBuilder, IHqlExpressionVisitor visitor)
+		{
+			var expression = visitor.Visit(targetObject).AsExpression();
+			var index = treeBuilder.Add(visitor.Visit(arguments[0]).AsExpression(), treeBuilder.Constant(1));
+			return treeBuilder.MethodCall("substring", expression, index, treeBuilder.Constant(1));
+		}
+	}
 
 	public class IndexOfGenerator : BaseHqlGeneratorForMethod
 	{

src/NHibernate/Util/ReflectHelper.cs

@@ -64,6 +64,21 @@ public static MethodInfo GetMethod<TSource>(Expression<Action<TSource>> method)
 			return ((MethodCallExpression)method.Body).Method;
 		}
 
+		/// <summary>
+		/// Extract the <see cref="MethodInfo"/> from a given expression.
+		/// </summary>
+		/// <typeparam name="TSource">The declaring-type of the method.</typeparam>
+		/// <typeparam name="TResult">The return type of the method.</typeparam>
+		/// <param name="method">The method.</param>
+		/// <returns>The <see cref="MethodInfo"/> of the method.</returns>
+		public static MethodInfo GetMethod<TSource, TResult>(Expression<Func<TSource, TResult>> method)
+		{
+			if (method == null)
+				throw new ArgumentNullException(nameof(method));
+
+			return ((MethodCallExpression) method.Body).Method;
+		}
+
 		/// <summary>
 		/// Extract the <see cref="MethodInfo"/> from a given expression.
 		/// </summary>