GetExpression 私有超载

对于一个过滤器：

这是创建查询的位置，它接收表达式参数和过滤器。

private static Expression GetExpression<T>(ParameterExpression param, QueryFilter queryFilter)
{
    // Represents accessing a field or property, so here we are accessing for example:
    // the property "Name" of the entity
    MemberExpression member = Expression.Property(param, queryFilter.PropertyName);

    //Represents an expression that has a constant value, so here we are accessing for example:
    // the values of the Property "Name".
    // Also for clarity sake the GetConstant will be explained in another example.
    ConstantExpression constant = GetConstant(member.Type, queryFilter.Value);

    // With these two, now I can build the expression
    // every operator has it one way to call, so the switch will do.
    switch (queryFilter.Operator)
    {
        case Operator.Equals:
            return Expression.Equal(member, constant);

        case Operator.Contains:
            return Expression.Call(member, ContainsMethod, constant);

        case Operator.GreaterThan:
            return Expression.GreaterThan(member, constant);

        case Operator.GreaterThanOrEqual:
            return Expression.GreaterThanOrEqual(member, constant);

        case Operator.LessThan:
            return Expression.LessThan(member, constant);

        case Operator.LessThanOrEqualTo:
            return Expression.LessThanOrEqual(member, constant);

        case Operator.StartsWith:
            return Expression.Call(member, StartsWithMethod, constant);

        case Operator.EndsWith:
            return Expression.Call(member, EndsWithMethod, constant);
    }

    return null;
}

对于两个过滤器：

它返回 BinaryExpresion 实例而不是简单的 Expression。

private static BinaryExpression GetExpression<T>(ParameterExpression param, QueryFilter filter1, QueryFilter filter2)
{
    // Built two separated expression and join them after.
    Expression result1 = GetExpression<T>(param, filter1);
    Expression result2 = GetExpression<T>(param, filter2);
    return Expression.AndAlso(result1, result2);
}