So this is the first time I've tried using IEqualityComparer and I'm running into an issue.
It's likely I just don't understand exactly what the code is doing behind the scenes.
The list I'm providing it looks like this:
Test Run | SN | Retest
1 185 0
2 185 1
3 185 1
4 185 1
I'm trying to use Distinct() to find the number of items which have a unique SN, and 'retest==1'.
var result = testRunList.Distinct(new UniqueRetests());
And the derived IEqualityCompare class looks like this:
public class UniqueRetests : IEqualityComparer<TestRunRecord>
{
// Records are equal if their SNs and retest are equal.
public bool Equals(TestRunRecord x, TestRunRecord y)
{
//Check whether any of the compared objects is null.
if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
return false;
//Check whether it's a retest AND if the specified records' properties are equal.
return x.retest == 1 && y.retest == 1 && x.boardSN == y.boardSN;
}
// If Equals() returns true for a pair of objects
// then GetHashCode() must return the same value for these objects.
public int GetHashCode(TestRunRecord record)
{
//Check whether the object is null
if (Object.ReferenceEquals(record, null)) return 0;
//Get hash code for the board SN field.
int hashRecordSN = record.boardSN.GetHashCode();
//Get hash code for the retest field.
int hashRecordRetest = record.retest.GetHashCode();
//Calculate the hash code for the product.
return hashRecordSN ^ hashRecordRetest;
}
}
Problem is that this seems to produce a which includes the first two items, whereas what I'm looking for is a list that includes only a single item where 'retest==1'.
Any idea what I'm doing wrong here? How is it that a record with 'retest == 0' is being returned?
Answer
If the condition is false, the objects are treated as if they are not equal. Distinct returns not-equal rows. Btw, you are violating the contract of IEqualityComparer with this type of code. The results are actually undefined. – usr
With "violating the contract" I mean (for example) than an object with retest==0 will compare unequal to itself. – usr
You need to filter out items with retest = 0. Put a .Where(x => x.retest != 0) in front of the Distinct.
Related
In C# .NET 4.0, I am struggling to come up with the most efficient way to determine if the contents of 2 lists of items contain any differences.
I don't need to know what the differences are, just true/false whether the lists are different based on my criteria.
The 2 lists I am trying to compare contain FileInfo objects, and I want to compare only the FileInfo.Name and FileInfo.LastWriteTimeUtc properties of each item. All the FileInfo items are for files located in the same directory, so the FileInfo.Name values will be unique.
To summarize, I am looking for a single Boolean result for the following criteria:
Does ListA contain any items with FileInfo.Name not in ListB?
Does ListB contain any items with FileInfo.Name not in ListA?
For items with the same FileInfo.Name in both lists, are the FileInfo.LastWriteTimeUtc values different?
Thank you,
Kyle
I would use a custom IEqualityComparer<FileInfo> for this task:
public class FileNameAndLastWriteTimeUtcComparer : IEqualityComparer<FileInfo>
{
public bool Equals(FileInfo x, FileInfo y)
{
if(Object.ReferenceEquals(x, y)) return true;
if (x == null || y == null) return false;
return x.FullName.Equals(y.FullName) && x.LastWriteTimeUtc.Equals(y.LastWriteTimeUtc);
}
public int GetHashCode(FileInfo fi)
{
unchecked // Overflow is fine, just wrap
{
int hash = 17;
hash = hash * 23 + fi.FullName.GetHashCode();
hash = hash * 23 + fi.LastWriteTimeUtc.GetHashCode();
return hash;
}
}
}
Now you can use a HashSet<FileInfo> with this comparer and HashSet<T>.SetEquals:
var comparer = new FileNameAndLastWriteTimeUtcComparer();
var uniqueFiles1 = new HashSet<FileInfo>(list1, comparer);
bool anyDifferences = !uniqueFiles1.SetEquals(list2);
Note that i've used FileInfo.FullName instead of Name since names aren't unqiue at all.
Sidenote: another advantage is that you can use this comparer for many LINQ methods like GroupBy, Except, Intersect or Distinct.
This is not the most efficient way (probably ranks a 4 out of 5 in the quick-and-dirty category):
var comparableListA = ListA.Select(a =>
new { Name = a.Name, LastWrite = a.LastWriteTimeUtc, Object = a});
var comparableListB = ListB.Select(b =>
new { Name = b.Name, LastWrite = b.LastWriteTimeUtc, Object = b});
var diffList = comparableListA.Except(comparableListB);
var youHaveDiff = diffList.Any();
Explanation:
Anonymous classes are compared by property values, which is what you're looking to do, which led to my thinking of doing a LINQ projection along those lines.
P.S.
You should double check the syntax, I just rattled this off without the compiler.
I have a very basic sql view which joins 3 tables: users, pictures, and tags.
How would one create the query below in a way that it won't list the same pictures more than once? In other words, I want to Group By pictures (I think) and return get the first insance of each.
I think this is very similar to the post Linq Query Group By and Selecting First Items, but I cannot figure out how to apply it in this case where the query is instantiating MyImageClass.
validPicSummaries = (from x in db.PicsTagsUsers
where x.enabled == 1
select new MyImageClass {
PicName = x.picname,
Username= x.Username,
Tag = x.tag }).Take(50);
To exclude duplicates, you can use the Distinct LINQ method:
validPicSummaries =
(from x in db.PicsTagsUsers
where x.tag == searchterm && x.enabled == 1
select new MyImageClass
{
PicName = x.picname,
Username= x.Username,
Tag = x.tag
})
.Distinct()
.Take(50);
You will need to make sure that the objects are comparable so that two MyImageClass objects that have the same PicName, Username, and Tag are considered equal (or however you wish to consider two of them as being equal).
You can write a small class that implements IEqualityComparer<T> if you would like to have a custom comparer for just this case. Ex:
private class MyImageClassComparer : IEqualityComparer<MyImageClass>
{
public bool Equals(MyImageClass pMyImage1, MyImageClass pMyImage2)
{
// some test of the two objects to determine
// whether they should be considered equal
return pMyImage1.PicName == pMyImage2.PicName
&& pMyImage1.Username == pMyImage2.Username
&& pMyImage1.Tag == pMyImage2.Tag;
}
public int GetHashCode(MyImageClass pMyImageClass)
{
// the GetHashCode function seems to be what is used by LINQ
// to determine equality. from examples, it seems the way
// to combine hashcodes is to XOR them:
return pMyImageClass.PicName.GetHashCode()
^ pMyImageClass.UserName.GetHashCode()
^ pMyImageClass.Tag.GetHashCode();
}
}
Then when you call distinct:
...
.Distinct(new MyImageClassComparer())
.Take(50);
I have method where I need to retrieve using EF the last ten entries in the database and check to see if there is a match between the value and the current term. Here is what I have thus far
public static int ValidatePassword(string username, string password, int securityUserId)
{
int validResult = 0;
/*Need to pass to client a value based upon success or failure of validation
* 0 - success
* 1 - password has already been used in the last 10 entries
* 2 - password does not meet CJIS requirements
*/
IEnumerable<string> oldpassword = null;
// Create a Regular Expression to determine whether or not special characters are present.
Regex regularExpression = new Regex("[^a-z0-9]");
//if id exists pull last ten passwords
if (securityUserId > 0)
{
long id = Convert.ToInt64(securityUserId);
using (var context = new SecurityEntities(string.Empty))
{
try
{
oldpassword = (from p in context.SecurityAudits
where p.SecurityUserId == id &&
p.OldPassword == password
orderby p.ActionDate descending
select p.OldPassword.Take(10).ToString()).ToList();
}
catch (Exception ex)
{
string err = string.Format("ValidateCJISPassword() was unable to return an object msg:{0}", ex.Message);
throw new Exception(err, ex.InnerException);
}
finally
{
context.Dispose();
}
}
}
else if (oldpassword == null)
{
//no matching record found now check other requirements
if ((password.Length >= DEFAULT_CJIS_PASSWORD_MIN_LENGTH) && regularExpression.IsMatch(password) && (password != username))
{
//success
validResult = 0;
}
else
{
//password does not meet standard CJIS requirements
validResult = 2;
}
}
else
{
//matching record was found
validResult = 1;
}
return validResult;
}
}
Where I am currently hung up is the query throws an exception on the ToString() method
LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.
I'm still learning EF and how linq works so I'm not sure what the best approach here is. Should I try to set the result to something other than IEnumerable like an array or List or is there another approach I should consider?
Thanks in advance,
Cheers,
Change this
oldpassword = (from p in context.SecurityAudits
where p.SecurityUserId == id &&
p.OldPassword == password
orderby p.ActionDate descending
select p.OldPassword.Take(10).ToString()).ToList();
To this
oldpassword = (from p in context.SecurityAudits
where p.SecurityUserId == id &&
p.OldPassword == password
orderby p.ActionDate descending
select p.OldPassword).Take(10).ToList();
The problem was that your Take(10) clause was not part of the whole result but inside the actual linq statement.. it goes on the outside of it to take the top 10 of the entire resultset.. then you do the ToList() which turns the whole thing into an array
The next problem is that you just created an array and assigned it to oldpassword
I don't see anything here that does anything with the array...
You need to do something like:
declare your array of strings
assign the array to the return of the linq query
evaluate the return for > 0 results
if > 0 then the password has been used in the last 10
if = 0 then new password should be ok, correct?
Now that I have an understanding of what I needed in the query I was able to also update the linq statement as follows:
var lastTenPassword = (from p in context.SecurityAudits.Take(10)
orderby p.ActionDate descending
where p.SecurityUserId == id
select p.OldPassword).ToList();
string oldpassword = lastTenPassword.Where(a => a == password).FirstOrDefault();
Testing is further down the line but now by moving the .Take() method inside the query I am explicitly grabbing the top ten where as my first attempt would have retrieved all the records and then grabbed the top ten.
For testing you can also see where I broke out the initial where() to first grab all records by id and then perform a filter on that set by looking for a matching password within that set.
Thanks again for your help
I had to write a method that does the following:
There is a DataSet let's say CarDataSet with one table Car and contains Primary key Id and one more column ColorId. And there is a string with Ids seperated with commas for example "5,6,7,8" (random length). The task is to check if all appropriate ColorIds are identical for given Car Ids.
For example:
String ids = "5,6,7,8"
If all the Cars ColorIds are for example 3,3,3,3 where the Car Ids are 5,6,7,8 then return true;
In other words - check if all cars with given Ids are in one color. Now I don't have my code anymore but I made this using 3 foreach loops and 3 linq expressions. Is there any simplier way to do this?
If you want all cars have same color means all of them should have same color as first one:
// first find the cars with given ids
var selectedCars = Cars.Where(x=>ids.Contains(x.ID.ToString());
// select one of them as comparer:
var firstCar = selectedCars.FirstOrDefault();
if (firstCar == null)
return true;
// check all of them has same color as first one:
return selectedCars.All(x=>x.ColorID == firstCar.ColorID);
Edit: Or if you have no problem with throwing exception when there is no car with given ids you can use two query in lambda syntax:
var selectedCars = Cars.Where(x=>ids.Contains(x.ID.ToString()));
return selectedCars.All(x=>x.ColorID == selectedCars.First().ColorID);
You could do this by performing a distinct, and asserting the count is 1.
var colors = Cars.Where(x=>ids.Contains(x.ID.ToString())
.Select(x=>x.ColorID)
.Distinct().Count();
return count == 1;
I get some data about customers in my database with this method:
public List<KlientViewModel> GetListOfKlientViewModel()
{
List<KlientViewModel> list = _klientRepository.List().Select(k => new KlientViewModel
{
Id = k.Id,
Imie = k.Imie,
Nazwisko = k.Nazwisko,
Nazwa = k.Nazwa,
SposobPlatnosci = k.SposobPlatnosci,
}).ToList();
return list;
}
but also I have another method which counts value for extra field in KlientViewModel - field called 'Naleznosci'.
I have another method which counts value for this field based on customers ids, it looks like this:
public Dictionary<int, decimal> GetNaleznosc(List<int> klientIds)
{
return klientIds.ToDictionary(klientId => klientId, klientId => (from z in _zdarzenieRepository.List()
from c in z.Klient.Cennik
where z.TypZdarzenia == (int) TypyZdarzen.Sprzedaz && z.IdTowar == c.IdTowar && z.Sprzedaz.Data >= c.Od && (z.Sprzedaz.Data < c.Do || c.Do == null) && z.Klient.Id == klientId
select z.Ilosc*(z.Kwota > 0 ? z.Kwota : c.Cena)).Sum() ?? 0);
}
So what I want to do is to join data from method GetNaleznosc with data generated in method GetListOfKlientViewModel. I call GetNaleznosc like this:
GetNaleznosc(list.Select(k => k.Id).ToList())
but don't know what to do next.
Having obtained the dictionary:
var dict = GetNaleznosc(list.Select(k => k.Id).ToList());
You can now efficiently look up the decimal value of Naleznosci for a given client:
foreach (var k in list)
k.Naleznosci = dict[k.Id];
Now you have merged the values into the main list. Is this what you mean?
By the way, in your function that builds the dictionary, you make it accept a List<int>, but then all you do is call ToDictionary on it, which only requires IEnumerable<int>. So change the parameter type to that, and then you can call it:
var dict = GetNaleznosc(list.Select(k => k.Id));
This removes the call to ToList, which avoids making an unnecessary intermediate copy of the whole list of Ids. Probably won't make much difference in this case if you're hitting a database and then building up a large set of results in memory, but maybe worth bearing in mind for other uses of these operations.
Also, looking again at the helper function, there is no apparent advantage in building up the set of results in a dictionary for a list of ids, because each one is handled independently. You could simply put:
public decimal GetNaleznosc(int klientId)
{
return (from z in _zdarzenieRepository.List()
from c in z.Klient.Cennik
where z.TypZdarzenia == (int) TypyZdarzen.Sprzedaz && z.IdTowar == c.IdTowar && z.Sprzedaz.Data >= c.Od && (z.Sprzedaz.Data < c.Do || c.Do == null) && z.Klient.Id == klientId
select z.Ilosc*(z.Kwota > 0 ? z.Kwota : c.Cena)).Sum() ?? 0);
}
That is, provide a function that discovers just one value. Now you can directly build the right list:
public List<KlientViewModel> GetListOfKlientViewModel()
{
return _klientRepository.List().AsEnumerable().Select(k => new KlientViewModel
{
Id = k.Id,
Imie = k.Imie,
Nazwisko = k.Nazwisko,
Nazwa = k.Nazwa,
SposobPlatnosci = k.SposobPlatnosci,
Naleznosci = GetNaleznosc(k.Id)
}).ToList();
}