NULL values

NULL T-SQL values are automatically converted into null .NET reference values and Nullable<T> struct values.

NULL parameter values can be specified by explicitly typing anonymous-object properties as type-null or Nullable<T>:

var result = await db.QueryAsync("UPDATE [SomeTable] SET [IntValue] = @Val WHERE [Id] = @Id",
	new
	{
		@Id = new Guid("cf9fad7a-9775-28b9-7693-11e6ea3b1484"),
		@Val = default(int?) // setting [IntValue] to NULL -- alternatively, "(int?)null"
	});

If parameter values are specified via Dictionary<string, (object, Type)> name-value mapping, use the .WithType() helper extension on the null Nullable<T> value to indicate NULL:

var result = await db.QueryAsync("UPDATE [SomeTable] SET [IntValue] = @Val WHERE [Id] = @Id",
	new Dictionary<string, object>
	{
		{ "@Id" , new Guid("cf9fad7a-9775-28b9-7693-11e6ea3b1484") },
		{ "@Val" , default(int?).WithType() } // setting [IntValue] to NULL
	});