Add proper support for nullable in Dapper #598 (#600)

Author: DarkWanderer

ClickHouse.Client.Tests/ORM/DapperTests.cs

@@ -207,4 +207,22 @@ public async Task ShouldWriteTwoFieldsWithTheSamePrefix()
         const string sql = "INSERT INTO test.dapper_prefixes (testField, testFieldWithSuffix) VALUES (@testField, @testFieldWithSuffix)";
         await connection.ExecuteAsync(sql, new { testField = 1, testFieldWithSuffix = 2 });
     }
+
+    [Test]
+    [TestCase(1.0)]
+    [TestCase(null)]
+    public async Task ShouldWriteNullableDoubleWithTypeInference(double? expected)
+    {
+        await connection.ExecuteStatementAsync("TRUNCATE TABLE IF EXISTS test.dapper_nullable_double");
+        await connection.ExecuteStatementAsync("CREATE TABLE IF NOT EXISTS test.dapper_nullable_double (balance Nullable(Float64)) ENGINE Memory");
+
+        var sql = @"INSERT INTO test.dapper_nullable_double (balance) VALUES (@balance)";
+        await connection.ExecuteAsync(sql, new { balance = expected });
+
+        var actual = await connection.ExecuteScalarAsync("SELECT * FROM test.dapper_nullable_double");
+        if (expected is null)
+            Assert.That(actual, Is.InstanceOf<DBNull>());
+        else
+            Assert.That(actual, Is.EqualTo(expected));
+    }
 }

ClickHouse.Client.Tests/Types/TypeMappingTests.cs

@@ -54,7 +54,7 @@ public class TypeMappingTests
     public Type ShouldConvertFromClickHouseType(string clickHouseType) => TypeConverter.ParseClickHouseType(clickHouseType, TypeSettings.Default).FrameworkType;
 
     [Test]
-    [TestCase(typeof(DBNull), ExpectedResult = "Nothing")]
+    [TestCase(typeof(DBNull), ExpectedResult = "Nullable(Nothing)")]
 
     [TestCase(typeof(sbyte), ExpectedResult = "Int8")]
     [TestCase(typeof(short), ExpectedResult = "Int16")]

ClickHouse.Client/Types/TypeConverter.cs

@@ -181,6 +181,7 @@ static TypeConverter()
 #endif
         ReverseMapping[typeof(DateTime)] = new DateTimeType();
         ReverseMapping[typeof(DateTimeOffset)] = new DateTimeType();
+        ReverseMapping[typeof(DBNull)] = new NullableType() { UnderlyingType = new NothingType() };
     }
 
     private static void RegisterPlainType<T>()