问题
In SQL, you can use the following syntax:
SELECT *
FROM MY_TABLE
WHERE VALUE_1 IN (1, 2, 3)
Is there an equivalent in C#? The IDE seems to recognise "in" as a keyword, but I don't seem to be able to find any information on it.
So, is it possible to do something like the following:
int myValue = 1;
if (myValue in (1, 2, 3))
// Do something
Instead of
int myValue = 1;
if (myValue == 1 || myValue == 2 || myValue == 3)
// Do something
回答1:
If you wanted to write .In then you could create an extension that allows you to do that.
static class Extensions
{
public static bool In<T>(this T item, params T[] items)
{
if (items == null)
throw new ArgumentNullException("items");
return items.Contains(item);
}
}
class Program
{
static void Main()
{
int myValue = 1;
if (myValue.In(1, 2, 3))
// Do Somthing...
string ds = "Bob";
if (ds.In("andy", "joel", "matt"))
// Do Someting...
}
}
回答2:
List.Contains() is I think what you're looking for. C# has in keyword and not an operator which serves completely different purpose then what you're referring in SQL.
There are two ways you can use in keyword in C#. Assume you have a string[] or List in C#.
string[] names; //assume there are some names;
//find all names that start with "a"
var results = from str in names
where str.StartsWith("a")
select str;
//iterate through all names in results and print
foreach (string name in results)
{
Console.WriteLine(name);
}
Referring your edit, I'd put your code this way to do what you need.
int myValue = 1;
List<int> checkValues = new List<int> { 1, 2, 3 };
if (checkValues.Contains(myValue))
// Do something
回答3:
You can do this:
var x = 99; // searched value
if (new[] {1,2,3,99}.Contains(x))
{
// do something
}
回答4:
You usually use the Contains method of a collection.
myCollection.Where(p => Enumerable.Range(1,3).Contains(p));
I hope it helps.
回答5:
There's no "in" operator in C#, the "in" keyword is used only with "foreach (... in ...)" or "from ... in ...".
The LINQ equivalent of your SQL query would be:
List<int> list = new List<int> { 1, 2, 3 };
var query = from row in my_table
where list.Contains(row.value1)
select row;
回答6:
Duplicate of : LINQ to SQL in and not in
select * from table where fieldname in ('val1', 'val2')
or
select * from table where fieldname not in (1, 2)
The equivalent of IN and NOT IN queries in LINQ to SQL would be something like this:
List<string> validValues = new List<string>() { "val1", "val2"};
var qry = from item in dataContext.TableName
where validValues.Contains(item.FieldName)
select item;
and this:
List<int> validValues = new List<int>() { 1, 2};
var qry = from item in dataContext.TableName
where !validValues.Contains(item.FieldName)
select item;
回答7:
I agree the best way to implement the In operator is with an Extension Method. I did it a little differently:
public static bool In(this string str, string CommaDelimintedStringSet)
{
string[] Values = CommaDelimintedStringSet.Split(new char[] { ',' });
foreach (string V in Values)
{
if (str == V)
return true;
}
return false;
}
The difference is that you don't have to put quotes around each value, only the entire set of comma delimited values, which is easier to type:
bool result = MyString.In("Val1,Val2,Val3");
回答8:
You can write an extension. I wrote one time ago, for making code like
if(someObject.stringPropertyX.Equals("abc") || someObject.stringPropertyX.Equals("def") || ....){
//do something
...
}else{
//do something other...
....
}
more readable with an extention s.t. one was able to write
if(someObject.stringPropertyX.In("abc", "def",...,"xyz"){
//do something
...
}else{
//do something other...
....
}
Here's the code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Some.Namespace.Extenders
{
public static class StringExtender
{
/// <summary>
/// Evaluates whether the String is contained in AT LEAST one of the passed values (i.e. similar to the "in" SQL clause)
/// </summary>
/// <param name="thisString"></param>
/// <param name="values">list of strings used for comparison</param>
/// <returns><c>true</c> if the string is contained in AT LEAST one of the passed values</returns>
public static bool In(this String thisString, params string[] values)
{
foreach (string val in values)
{
if (thisString.Equals(val, StringComparison.InvariantCultureIgnoreCase))
return true;
}
return false; //no occurence found
}
}
}
This is the one specific to my needs at that time, but you may adapt and modify it to match more different types.
回答9:
For digits from 0 to 9:
"123".Contains(myValue)
For any other Stuff:
"|1|2|3|".Contains("|" + myValue + "|")
回答10:
For your updated question, you could also use a switch-statement.
switch (myvalue)
{
case 1:
case 2:
case 3:
// your code goes here
break;
}
回答11:
There is no in operator that looks for a value in a collection, instead it's a method of the collection, called Contains.
The most scalable solution is to use a HashSet as the collection. Checking for a value in a HashSet is close to an O(1) operation, compared to doing it in a List where it is an O(n) operation. That means that you can pack a lot of values in a HashSet and it's still fast, while looking for a value in a List gets slower the more values you have.
Example:
var set = new HashSet<int>();
set.Add(1);
set.Add(2);
set.Add(3);
var result = items.Select(i => set.Contains(i.value));
回答12:
Common, LINQ way more powerful:
var list = new List<string> { "Tomato", "Orange", "Mango"};
var query = from i in my_table
from v in list
where i.Name.StartsWith(v)
select i;
回答13:
The in keyword in C# is for the foreach statement and for LINQ query expressions. There is no functionality equivalent to SQL's in operator in C# per se, but LINQ offers similar functionality with Contains().
var list = {1, 2, 3}
var filtered = (
from item in items
where list.Contains(item)
select item).ToArray().
回答14:
I do something like this:
var shippingAddress = checkoutContext.Addresses.Where(a => (new HashSet<AddressType> { AddressType.SHIPPING_ONLY, AddressType.BILLING_AND_SHIPPING }).Contains(a.AddressType) && a.Id == long.Parse(orderDto.ShippingAddressId)).FirstOrDefault();
来源：https://stackoverflow.com/questions/3164998/is-there-a-c-sharp-in-operator