c# – 当查询仅由属性不同时,如何解决LINQ to Entity查询重复？

2019年7月28日 132次阅读

我有两个DbSets,Foo和Bar. Foo有一个标识字符串属性FooName,而Bar有一个标识字符串属性BarName.

我正在设计一个非常简单的搜索功能,其中用户的查询字词可以等于或包含在识别名称中.

所以我有两种方法(大大简化)：

public ActionView SearchFoo(string query) 
{
    var equalsQuery = db.Foo.Where(f => f.FooName.Equals(query));
    var containsQuery = db.Foo.Where(f => f.FooName.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}


public ActionView SearchBar(string query) 
{
    var equalsQuery = db.Bar.Where(f => f.BarName.Equals(query));
    var containsQuery = db.Bar.Where(f => f.BarName.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}

显然我想要一些辅助方法,如下所示：

public IList<T> Search<T>(string query, DbSet<T> set) 
{
    var equalsQuery = set.Where(f => ???.Equals(query));
    var containsQuery = set.Where(f => ???.Contains(query)).Take(10); // Don't want too many or else a search for "a" would yield too many results

    var result = equalsQuery.Union(containsQuery).ToList();
    ... // go on to return a view
}

我最初尝试添加一个Func< T,string>到搜索参数,我可以使用f => f.FooName和b => b.BarName,但LINQ to Entities在执行查询期间不支持lambda表达式.

我一直在摸索如何提取这种重复.

最佳答案 您可以使用Expression< Funt< T,string>>实现此目的.

public IList<T> Search<T>(string query, DbSet<T> set, Expression<Func<T, string>> propExp)
{
    MethodInfo method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
    ConstantExpression someValue = Expression.Constant(query, typeof(string));
    MethodCallExpression containsMethodExp = 
             Expression.Call(propExp.Body, method, someValue);
    var e = (Expression<Func<T, bool>>)
              Expression.Lambda(containsMethodExp, propExp.Parameters.ToArray());
    var containsQuery = set.Where(e).Take(10);

    BinaryExpression equalExpression = Expression.Equal(propExp.Body, someValue);
    e = (Expression<Func<T, bool>>)
                 Expression.Lambda(equalExpression, propExp.Parameters.ToArray());

    var equalsQuery =  set.Where(e);

    var result = equalsQuery.Union(containsQuery).ToList();
}

那你就叫它：

Search ("myValue", fooSet, foo=>foo.FooName);

如果您可以使用静态方法,那么您可以将其作为扩展方法：

public static IList<T> Search<T>(this DbSet<T> set, 
                                  string query, Expression<Func<T, string>> propExp)

并称之为：

FooSet.Search ("myValue", foo=>foo.FooName);
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