如何使用.net实现SQL IN()的等效功能

using System;
using System.Linq;

static class SqlStyleExtensions
{
    public static bool In(this string me, params string[] set)
    {
       return set.Contains(me);
    }
}

用法:

if (Variable.In("AC", "BC", "EA"))
{

} 

/// <summary>
/// Returns true if the value is represented in the provided enumeration.
/// </summary>
/// <typeparam name="T">Type of the value</typeparam>
/// <param name="obj">The object to check if the enumeration contains</param>
/// <param name="values">The enumeration that might contain the object</param>
/// <returns>True if the object exists in the enumeration</returns>
public static bool In<T>(this T obj, IEnumerable<T> values) {
    return values.Contains(obj);
}

编辑: 不过,我会在这里邮寄,因为这是一个更通用的版本。

public static IQueryable<T> WhereIn<T, TValue>(
                this IQueryable<T> query,
                Expression<Func<T, TValue>> selector, 
                params TValue[] collection) where T : class
{
    if (selector == null) throw new ArgumentNullException("selector");
    if (collection == null) throw new ArgumentNullException("collection");
    ParameterExpression p = selector.Parameters.Single();

    if (!collection.Any()) return query;

    IEnumerable<Expression> equals = collection.Select(value =>
       (Expression)Expression.Equal(selector.Body,
            Expression.Constant(value, typeof(TValue))));

    Expression body = equals.Aggregate(Expression.Or);
    return query.Where(Expression.Lambda<Func<T, bool>>(body, p));
}

其中:

public static IQueryable<T> WhereNotIn<T, TValue>(
                this IQueryable<T> query, 
                Expression<Func<T, TValue>> selector, 
                params TValue[] collection) where T : class
{
    if (selector == null) throw new ArgumentNullException("selector");
    if (collection == null) throw new ArgumentNullException("collection");
    ParameterExpression p = selector.Parameters.Single();

    if (!collection.Any()) return query;

    IEnumerable<Expression> equals = collection.Select(value =>
       (Expression)Expression.NotEqual(selector.Body,
            Expression.Constant(value, typeof(TValue))));

    Expression body = equals.Aggregate(Expression.And);

    return query.Where(Expression.Lambda<Func<T, bool>>(body, p));
}

用法:

var args = new [] { 1, 2, 3 };
var bookings = _repository.Find(r => r.id > 0).WhereIn(x => x.BookingTypeID, args);
// OR we could just as easily plug args in as 1,2,3 as it's defined as params
var bookings2 = _repository.Find(r => r.id > 0).WhereIn(x => x.BookingTypeID, 1,2,3,90);

var bookings3 = _repository.Find(r => r.id > 0).WhereNotIn(x => x.BookingTypeID, 20,30,60);

这真的让我每次复习都会笑:)

吉姆

-最初来源于此处,但修改为使用iqueryable和params: 'Contains()' workaround using Linq to Entities?

if((new int[] {1, 2, 4, 7}).Contains(value))
{
    // Do some work.
}

public static bool In<T>(T this obj, IEnumerable<T> col)
{
    return col.Contains(obj);
}

最初的例子是:

if(value.In(new int[] {1, 2, 4, 7}))
{
    // Do some work.
}

System.Linq ...

(VB.NET)

Enumerable.Contains({1, 2, 4, 7}, value)

{1, 2, 4, 7}.Contains(value)

(C#)

Enumerable.Contains(new int[]{1, 2, 4, 7}, value);

new int[] {1, 2, 4, 7}.Contains(value);

    int myValue = 1;
    List<int> checkValues = new List<int> { 1, 2, 3 };

    if (checkValues.Contains(myValue))
        // Do something 

使用LINQ

var q = from x in collection
        where (new int[] { 1, 2, 4, 7}).Contains(x.value)
        select x

HashSet<T> .

HashSet<int> numbers = new HashSet<int> { 1, 2, 4, 7 };
bool is5inSet = numbers.Contains(5);

int[] values = new int[]{1, 2, 4, 7};
int target = 2;
bool contains = values.Any(v => v == target);

或使用 .Contains 正如一些人所建议的那样。