Question

我有一个搜索表单，允许用户以几种不同的方式搜索几个不同的字段。这是我的代码示例。

var claims = from c in db.Claims select c;

switch (ddlSearchField.Text)
{
    case "StartsWith":
        claims = claims.Where(c => c.companyFileID.StartsWith(txtSearchBox.Text));
        break;

    case "Equals":
        claims = claims.Where(c => c.companyFileID == txtSearchBox.Text);
        break;

    case "Contains":
        claims = claims.Where(c => c.companyFileID.Contains(txtSearchBox.Text));
        break;
}

我有大约十个不同的字段，用户可以搜索，所以我的外部开关语句非常大。必须有一种更优雅的方式来实现这一目标。

Answer 1

您可以通过创建扩展方法来重构代码的某些部分。这样的事情：

static class QueryableExtensions
{
    private static MethodInfo StringContainsMethod;
    private static MethodInfo StringStartsWithMethod;

    static QueryableExtensions()
    {
        Type[] singleStringParam = new[] {typeof(string)};
        StringContainsMethod = typeof(string).GetMethod("Contains", singleStringParam);
        StringStartsWithMethod = typeof(string).GetMethod("StartsWith", singleStringParam);
    }

    public static IQueryable<T> AppendTextFilter<T>(this IQueryable<T> queryable, Expression<Func<T, string>> memberSelector, string condition, string value)
    {
        Expression expression = null;
        switch (condition)
        {
            case "StartsWith":
                expression = Expression.Call(
                                memberSelector.Body,
                                StringStartsWithMethod,
                                Expression.Constant(value));
                break;

            case "Equals":
                expression = Expression.Equal(
                                memberSelector.Body,
                                Expression.Constant(value));
                break;

            case "Contains":
                expression = Expression.Call(
                                memberSelector.Body,
                                StringContainsMethod,
                                Expression.Constant(value));
                break;

            default:
                throw new NotSupportedException(string.Format("'{0}' is not a supported condition", condition));
        }

        var lambda = Expression.Lambda<Func<T, bool>>(
                        expression,
                        memberSelector.Parameters);
        return queryable.Where(lambda);
    }
}

然后你可以这样使用它：

var claims = db.Claims
             .AppendTextFilter(c => c.companyFileID, ddlSearchField.Text, txtSearchBox.Text)
             .AppendTextFilter(c => c.someOtherProperty, ddlOtherSearchField.Text, txtOtherSearchBox.Text)
             ...;

Answer 2

您是否尝试过动态LINQ？它允许您构建类似于SQL WHERE子句的基于字符串的查询语句。

http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

Answer 3

您可以制作可能谓词的字典：

Dictionary<string, Func<string, Expression<Func<Claim, bool>>>> map = new Dictionary<string, Func<string, Expression<Func<Claim, bool>>>>() {
    { "StartsWith", t => c => c.companyFileID.StartsWith(t) },
    { "Equals",     t => c => c.companyFileID == t },
    { "Contains",   t => c => c.companyFileID.Contains(t) }
};

可以这样使用：

var search = ddlSearchField.Text;
var text = txtSearchBox.Text;

var claims = from c in db.Claims select c;

Func<string, Expression<Func<Claim, bool>>> predicate = null;
if(dict.TryGetValue(search, out predicate))
    claims = claims.Where(predicate(text));

ASP.NET搜索表单 - 动态Linq到SQL？

3 个答案: