Question

如何将带有查询的字符串转换为实际的LINQ表达式，以便执行它？

if(email!= null)
{
    query += "x => x.Email.Contains(email)";
}
if (firstname != null)
{
    query += "&& x.FirstName.Contains(firstname)";
}
if (lastname != null)
{
    query += "&& x.LastName.Contains(lastname)";
}
return context.UserAccounts.Where(query).ToList();

Answer 1

尝试一下：

<your collection>.
    .Where(o => (email!= null)? o.Email.Contains(email) : true).
    .Where(o => (firstname != null)? o.FirstName.Contains(firstname) : true).
    .Where(o => (lastname != null)? o.LastName.Contains(lastname) : true).
    ToList();

基本上，这意味着过滤器仅在满足条件时才适用（就像if语句中一样）。

Answer 2

您可以将查询另存为IQueryable（来自System.Linq名称空间），而不是将查询另存为字符串。

var query = context.UserAccounts;
if(email!= null)
{
    query = query.Where(x => x.Email.Contains(email));
}

if (firstname != null)
{
    query = query.Where(x => x.FirstName.Contains(firstname));
}

if (lastname != null)
{
    query = query.Where(x => x.LastName.Contains(lastname));
}
return query.toList();

当您最终致电ToList()时，IQueryable被枚举为List中的UserAccount。

Answer 3

如果需要使用where子句的组合进行查询，则每次结果都需要保存在查询本身中

var query = context.UserAccounts;

if(email != null)
{
    query = query.Where(x => x.Email.Contains(email));
}
if (firstname != null)
{
    query = query.Where(x => x.FirstName.Contains(firstname));
}
if (lastname != null))
{
    query = query.Where(x => x.LastName.Contains(lastname));
}
return query.ToList();

另一种复杂的方法可能是使用Actions

Answer 4

也许我没有正确看到您的问题，所以如果我错了，请原谅。为什么可以将字符串转换为LINQ表达式：

var query = context.UserAccounts;

if(email != null)
{
    query = query.Where(x => x.Email.Contains(email));
}
if (firstname != null)
{
    query = query.Where(x.FirstName.Contains(firstname));
}
if (lastname != null))
{
    query = query.Where(x.LastName.Contains(lastname));
}
return query.toList();

字符串到LINQ表达式？

4 个答案: