My Code:
var lastName = employees
.Where(a => a.Number ==
(dm.MTM.Where(b => b.MTT.IsManager)
.Select(c => c.Number)
.FirstOrDefault()))
.Select(z => z.LastName)
.FirstOrDefault();
Error Message:
Unable to create a constant value of type 'XXX.Models.Mt.MTM'. Only primitive types or enumeration types are supported in this context.
Try:
int? num = dm.MTM.Where(b => b.MTT.IsManager).Select(c => c.Number).FirstOrDefault();
var lastName = employees.Where(a => a.Number == num).Select(z => z.LastName).FirstOrDefault();
But you should add a check
if (num == null)
{
// bad things, don't execute second query
}
between the two instructions.
The error is because in an Entity Framework query you can't do "things" too much fancy, like the things necessary to calculate num.
Try:
// Calculate the number outside of the main query
// because the result is fixed
var nb = dm.MTM
.Where(b => b.MTT.IsManager)
.Select(c => c.Number)
.FirstOrDefault();
// Perform the main query with the number parameter already calculated before
string lastName = String.Empty;
if (nb != null) // if null no need to run the query
{
lastName = employees
.Where(a => a.Number == nb)
.Select(z => z.LastName)
.FirstOrDefault();
}
You don't need to get the number from the database for each employee, calculate the value before running your main query.
This will be faster, less error-prone and better for caching.
Related
I want to get the distinct list of studentname from the database who are still active. Ihis is what I've tried.
string strJSON = JsonConvert.SerializeObject(
context.Students.Where(x => x.IsActive == 1).Distinct().ToList());
Distinct function works on all columns, so I assume that you want only student name.
string strJSON = JsonConvert.SerializeObject(
context
.Students
.Where(x => x.IsActive==1)
.Select(x=>x.StudentName)
.Distinct()
.ToList()
);
I have a LINQ query where I want to select and read the p.Api value.
var api = DataAccessNew.Instance.dcServers.Where(p => p.Ip == IpAddress).Select(p => p.Api);
How do I read the p.Api value?
I have tried api.ToString() but I get SQL instead of actual column value.
You are getting an IEnumerable<> back (and your ToString call is showing you the value of that expression).
If you are expecting a single value, do this:
var api = DataAccessNew.Instance.dcServers
.Where(p => p.Ip == IpAddress)
.Select(p => p.Api)
.Single();
You might be interested to read about the other methods like Single(): SingleOrDefault, First, FirstOrDefault. Which one you used depends on whether you are expecting a single or multiple values returned (Single vs. First) and what you want to happen if there are no values (the *Default methods will return the type default instead of throwing an exception).
Or if you want to look at all the returned values:
var api = DataAccessNew.Instance.dcServers
.Where(p => p.Ip == IpAddress)
.Select(p => p.Api);
foreach (var apiValue in api)
{
// apiValue will have the value you're looking for.
}
Try this snippet of code:
string apiValue = api.FirstOrDefault().ToString();
your syntex seems ok..
By the way try this
string api =DataAccessNew.Instance.dcServers.Where(p => p.Ip == IpAddress).Select(p => p.Api).FirstOrDefault();
if p.Ip is a unique key in your table you could try to add .FirstOrDefault() after your Linq query.
public string getselectedvalue(ListBox l)
{
string vtext="",vval="";
var selectedQueryText = l.Items.Cast<ListItem>().Where(item => item.Selected);
var selectedQueryVal = l.Items.Cast<ListItem>().Where(item => item.Selected).Select(item => item.Value);
vtext= String.Join("','", selectedQueryText ).TrimEnd();
vval= String.Join("','", selectedQueryVal ).TrimEnd();
return v;
}
I am writing a lambda in linq for getting the pivoted data from the resulting list.For getting the pivoting columns am setting a where condion to get the value.the problem here is am getting default value if the where condtion fails.I dont want the column if the where condition fails.Kindly help me out.
var query = dataList
.GroupBy(c => c.IpAddress)
.Select(g => new
{
IPAddress = g.Key,
AssetType = g.ElementAtOrDefault(0).AssetTypeName,
AssetName = g.ElementAtOrDefault(0).AssetName,
//if where condition fails i dont need this column.
//also am giving c.DsName == "Active Calls" ,how to achieve tis dynamically
**ActiveCalls = g.Where(c => c.DsName == "Active Calls").Sum(c => c.CurrentValue),**
**HoldCalls = g.Where(c => c.DsName == "Hold Calls").Sum(c => c.CurrentValue),**
**CPU = g.Where(c => c.DsName == "CPU").Sum(c => c.CurrentValue),**
});
Why not just create a value to hold the sum and then specify its a type in another column. That way you don't have to deal with null columns. (The assumption is that only one type is valid at a time).
Value = g.Sum(c => c.CurrentValue), // Value as specified by the the DsName property.
DsName = c.DsName
I got this error when i use sum function in LINQ:
The cast to value type 'Decimal' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.
GroupProduct.Where(a => a.Product.ProductID==1).Sum(Content => Content.Amount==null?0:Content.Amount),
This is what I usually use. This will cover the possibility of Amount being null and also cover the possibility of an empty set.
GroupProduct.Where(a => a.Product.ProductID == 1)
.Select(c => c.Amount ?? 0) // select only the amount field
.DefaultIfEmpty() // if selection result is empty, return the default value
.Sum(c => c)
DefaultIfEmpty() returns the default value associated with Amount's type, which is int, in which case the default value is 0.
Did you try the following:
GroupProduct.Where(a => a.Product.ProductID==1).Sum(Content => (decimal?)Content.Amount)
The code from my application looks like:
var data = query.AsEnumerable().Select(row => TaskInfo.FetchTaskInfo(row,
ctx.ObjectContext.Hours.Where(hour => hour.TaskId == row.TaskId).Sum(hour => (decimal?)hour.Duration),
ctx.ObjectContext.Notes.Count(note => note.SourceType == (int)SourceType.Task && note.SourceId == row.TaskId)));
You could exclude at source?
var sum = GroupProduct.Where(a => a.Product.ProductID==1 && a.Amount != null)
.Sum(a => (decimal)a.Amount);
Try this:
var sum = GroupProduct.Where(a => a.Product.ProductID==1).Sum(Content => (int?) Content.Amount);
sum = sum ?? 0;
This looks like it should work (and usually does) but fails when the Where() method returns null:
decimal sum1 = GroupProduct
.Where(a => a.Product.ProductID == 1)
.Sum(c => c.Amount ?? 0);
The error: "The cast to value type 'Decimal' failed because the materialized value is null" is due to the Sum() method returning null (not zero) when summing over an empty set.
Either of these work for me:
decimal? sum2 = GroupProduct
.Where(a => a.Product.ProductID == 1)
.Sum(c => c.Amount);
decimal sum3 = GroupProduct
.Where(a => a.Product.ProductID == 1)
.Sum(c => c.Amount) ?? 0;
I have IQueryable list of objects of type T which I want to transform into objects of type K
List<K> tranformedList = originalList.Select(x => transform(x)).ToList();
the transform function returns null if it cannot tranform the objects.If I want to filter out null elements can I call
List<K> tranformedList = originalList.Select(x => transform(x))
.Where(y => y != default(K))
.ToList();
or is there any other way of filtering out null elements when calling Select in LINQ ?
Can't you just do something like this:
List<K> tranformedList = originalList.Select(x => tranform(x))
.Where(y => y != null) //Check for nulls
.ToList();
What about
List<K> tranformedList = originalList
.Select(transform)
.OfType<K>()
.ToList()
Takes care of unboxing an getting rid of nulls at the same time (especially when K is a struct)
David B I dont believe you that your code .Where(y => y != null) works when K is an int! There is NO WAY you will get that code to compile if K is an int!
List<K> tranformedList = originalList.Select(x => transform(x))
.Where(y => !string.IsNullOrEmpty(y))
.ToList();
After your Select linq query, filter null values with !string.IsNullOrEmpty("string") or string.IsNullOrWhiteSpace("string") in your Where query.
Use Where Instead of Select (Linq).
Where returns the list without null values directly
List tranformedList = originalList.Where(x => x != null).ToList();
You could try a for loop and add the non nulls to the new transformed list.
foreach (var original in originalList)
{
K transformed = tranform(orignal);
if (transformed != null)
{
tranformedList.Add(transformed);
}
}
or you could try
List<K> tranformedList = (from t in
(from o in originalList
select tranform(o))
where t != null
select t).ToList();
I think Nathan's works as well but is less verbose