I am trying to amend a c# project. I am a vb.vet programmer so having a few issues as I am new to linq. I am trying to run a Linq to Entity query. I want to select the MapEast where town = town. I keep get an error The specified cast from a materialized System.Decimal' type to the 'System.Int32' type is not valid.. I would like to put a max(1) in here too so it returns only the highest number.
var topEast = 0;
try
{
topEast = this._uow.Addresses
.Where(a =>
a.Town.Trim().ToUpper() == town.Trim().ToUpper())
.Select(m => m.MapEast).FirstOrDefault ();
return -1;
}
catch
{
return -1;
}
Thanks
var is used for implicitly typed local variable. When you defined var topEast = 0;, topEast was implicitly assigned type int, and not decimal as per your query. You can fix it by explicitly defining topEast as decimal.
decimal topEast = 0;
I would like to put a max(1) in here too so it returns only the
highest number.
Not really sure what you are trying to return, because you are returning -1 from try as well as catch block. If you are trying to return the Max value of MapEast field then you will need Enumerable.Max, otherwise FirstOrDefault would return the first item or null based on criteria.
Related
public ActionResult Votation(int id=0)
{
var events = db.Events_Info_tbl.Where(x => x.is_active == true).FirstOrDefault();
//query the first category
List<CampaignManager_tbl> candidates = new List<CampaignManager_tbl>();
candidates = (from cat in db.Events_Category_tbl
join can in db.Candidates_Info_tbl
on cat.events_category_id equals can.events_category_id
where cat.events_info_id == events.events_info_id
select new CampaignManager_tbl {
events_category_name = cat.events_category_name,
candidates_fullname = can.candidates_fullname,
candidates_info_id = can.candidates_info_id,
vote_no = cat.vote_no.Value,
isSelected = can.isSelected.Value,
events_category_id = cat.events_category_id
}).ToList();
return View(candidates);
}
This code was working before but now I've got this error: The cast to value type 'Boolean' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.
What's wrong with this? I didn't change any of my codes before. It just this time I've got an error.
I ran into this error because of a different problem. In my query I am selecting specific columns from an account member and its associated account:
DBContext.Set<AccountMember>()
.Include(am => am.Account)
.Select(am => new
{
am.ID,
am.IsPrimary,
Account = new { am.Account.ID, am.Account.DoNotEmail }
});
Both of my boolean properties, IsPrimary and DoNotEmail are non-nullable so the resulting property type of their properties in the anonymous type is bool. The exception occurs when an AccountMember does not have an Account. Since am.Account is null, the expression am.Account.DoNotEmail returns null. But the anonymous type's DoNotEmail property inferred it was of type bool which cannot be set to null. The solution is to give it a hint that this should be a nullable bool by casting the property to a bool?:
DBContext.Set<AccountMember>()
.Include(am => am.Account)
.Select(am => new
{
am.ID,
am.IsPrimary,
Account = new { am.Account.ID, DoNotEmail = (bool?)am.Account.DoNotEmail }
});
Hapily EF is smart enough to ignore this cast and not blow up trying to convert it to SQL.
I'm going out on a limb here, but I'd guess it has to do with this line:
isSelected = can.isSelected.Value,
If when the two tables are joined, one of them does not return a value for that particular field, it will be null. It appears the type here is a nullable of some type - probably bool?, based on the naming - and that would match your error.
Try to replace it with the following:
isSelected = can.isSelected.HasValue ? can.isSelected.Value : false,
Of course, you can replace the default false here with true, if that makes more sense in your case.
Edit: Note that you should probably do the same for the other column you use in a similar way (I'm assuming this is returning an int?):
vote_no = cat.vote_no.HasValue ? cat.vote_no.Value : 0
Is db.Candidates_Info_tbl.IsSelected a bit field?
If so, the query could likely result in
isSelected = can.isSelected.Value,
with isSelected being null, whereas you probably declared isSelected as bool in CampaignManager_tbl class.
Change the isSelected to bool in the CampaignManager_tbl class and try again.
if your field type is int while some value is null, then you can use case-when and convert to bit like below
SELECT CASE WHEN FieldName IS NOT NULL THEN CONVERT(BIT,FieldName) ELSE CONVERT(BIT,0) END AS NewFieldName
I have a SqlParameter contained in a var coming back from a stored procedure and the value is either Null, an int or -1. How do cast the var to check if it is -1 in C#?
var p_eventID = new SqlParameter()
{
ParameterName = "#EventID",
Value = (eventID.HasValue
? (object)eventID.Value
: (object)System.Data.SqlTypes.SqlInt32.Null),
Direction = ParameterDirection.InputOutput
};
I have tried:
if ((int?)eventID.Value == -1)
But it gives me an invalid cast error.
SQL requests report NULLs to C# through DBNull objects, forcing the cast to fail.
You can do the other kind of cast to int? without triggering an error:
var eventIdInt = eventId as int?;
if (eventIdInt == -1) {
...
}
Rather than throwing an exception, the x as type style of cast would produce a null value when the cast fails.
Your approach when an error occurs, it will throw an Invalid Cast Exception. However, if you utilize the as int if it fails it will return a null. Which will be a bit more graceful, and forgiving.
So you could do the following:
if(eventId != null)
{
// Do Stuff.
}
So that returned value from your scalar, should either contain an actual int or a null.
So in order to solve why you receive the error with the (int) we would need to see how you implement the scalar and know the type of object it is actually returning before the cast.
public List<Workflow> GetMyWorkflows(int[] MyRoles)
{
int[] myWorkflowIDs = new int[] { };
RapidWorkflowDataContext context = new RapidWorkflowDataContext();
var query = from w in context.WorkflowRoles
where MyRoles.Contains((int)w.RoleID)
select w.WorkflowID;
var distinctWorkflows = query.Distinct();
myWorkflowIDs = distinctWorkflows.toArray();
return myWorkflowIDs;
}
In this method I want to retrieve an array of workflows that a user can
access.
I get the following error : Cannot implicitly convert type 'int?[]' to 'int[]'
I want to retrieve an array of workflows
But your method must return a List<Workflow> or a List<int>.
So you should skip the array idea. The other issue is between int and int?. You can solve that in the select clause with select w.WorkflowID.Value or select w.WorkflowID ?? 0. Or simply select w for a List<Workflow>.
Also it is a good idea to dispose a context when it becomes unreachable.
public List<int> GetMyWorkflows(int[] MyRoles)
{
using (RapidWorkflowDataContext context = new RapidWorkflowDataContext())
{
var query = from w in context.WorkflowRoles
where MyRoles.Contains((int)w.RoleID)
select w.WorkflowID ?? 0;
// select w; to return a List<WorkFlow>
var distinctWorkflows = query.Distinct();
return distinctWorkflows.ToList(); // ToList because we are closing the Context
}
}
I'm going to guess that WorkflowID is of type int?. If you are certain that it cannot be null, change your central query to:
var query = from w in context.WorkflowRoles
where MyRoles.Contains((int)w.RoleID)
select w.WorkflowID.Value;
This will ensure that query is now of type IEnumerable<int> instead of IEnumerable<int?>, with the int following on throuhh the Distinct() and ToArray() functions.
This seems like a pretty good error to me
Cannot convert type 'int?[]' to 'int[]'
You must have an array of type int? and be trying to implicitly convert it to int.
Therefore you have two options - stop trying to implicitly convert, and allow the result to be int?[], like this:
int?[] myWorkflowIDs = new int?[] { };
or force the convert to take place, like this:
RapidWorkflowDataContext context = new RapidWorkflowDataContext();
var query = from w in context.WorkflowRoles
where MyRoles.Contains((int)w.RoleID)
select (int)w.WorkflowID;
// or w.WorkflowID ?? 0; as necessary
So int? can also be written Nullable<int> which is basically an int that can take null values. For example:
int? nullableNumber = 5; // Set to a value
nullableNumber = null? // Set to null (which is possible because int? is nullable).
As you can imagine, Nullable<int> is useful for databases because sometimes you might have a column that has null values, and so this type gives a useful means of mapping to this sort of value. The problem, though is that in your code you have to deal with two different types, int vs. int?. You can cast between the two values by using:
// If the nullable-integer is not-null then use it's value, else default to `0`.
int nonNullable = nullableNumber ?? 0;
which will replace nulls with 0 if the value is null. Or you can just store your myWorkflowIDs in a nullable value (Nullable<int>[] or int?[]), which semantically better reflects what the column value in the database actually is.
I have a Linq query I need to use in my Index method in the page's controller, however I am getting the following error on the "select new" portion of the code:
Error
Cannot implicitly convert type 'System.Linq.IQueryable<AnonymousType#1>' to 'string'
Action method
public ActionResult Index(string query)
{
var agentProductTraining = "";
if (String.IsNullOrEmpty(query))
{
BlankIndex();
}
else
{
agentProductTraining = from course in db.Course
where
course.CourseDescription.Contains(query)
select new
{
course.CourseCode,
course.CourseDescription,
course.Partner,
course.Status,
course.LastChangeDate,
course.LastChangeOperator
};
}
return View(agentProductTraining.ToList());
}
As the error clearly states, you cannot assign the result of a LINQ query (IQueryable<T>) to a variable of type string.
You should declare the variable in that line:
var agentProductTraining = select ...
You've initialized the variable as a string, so the compiler makes the variable a string type (since you've user the var keyword), but then tried to assign a collection of anonymous types to it.
You can declare it as an object instead or var:
object agentProductTraining; // can safely be overwritten
Also I assume you mean:
return BlankIndex();
in the if block. Otherwise it will fall through to
return View(agentProductTraining.ToList());
where agentProductTraining is going to be null
Of course if you use return BlankIndex in the if block you can simplify the whole thing:
if (String.IsNullOrEmpty(query))
{
return BlankIndex();
}
// don't need an `else` here since the if will return to the caller
var agentProductTraining = from course in db.Course
where
course.CourseDescription.Contains(query)
select new
{
course.CourseCode,
course.CourseDescription,
course.Partner,
course.Status,
course.LastChangeDate,
course.LastChangeOperator
};
return View(agentProductTraining.ToList());
Suppose i have this sql statement and I have executed a sql command to get a datareader:
"select 1 union select 2"
//.....
var rdr = cmd.ExecuteReader();
and now i want to read the value in the first column of the first row:
var myInt = (int)rdr.GetValue(0); //great this works
var myLong = (long)rdr.GetValue(0); //throws cast exception, even though you can cast int to long
So it appears the type you cast to in C# needs to match exactly the SQL type. I.E. If the sql type is bigint, you can only cast to long. If the sql type is int, you can only cast to int. No mix and match...
I just want to get something that works regardless of the type of integer c# asks for and sql returns, as long as you could theoretically cast one to the other. So if SQL Server gives me a floating point type, and I'm asking for an int, I want the truncated int you get from doing that cast.
My goal is to make this work with generics, so I can have this function work when the generic parameter doesn't exactly match the datatype in sql server:
List<T> GetFirstColumn<T>(string sql) where T : struct
{
//get connection, execute reader
// loop:
// lst.Add( (T) rdr.GetValue(0));
}
I'd like this to work for both statments:
var sql1 = "Select 1"; //sql int
var sql2 = "Select cast(1 as bigint)"; //sql equivalent of a long
var lst1 = GetFirstColumn<int>(sql1);
var lst2 = GetFirstColumn<int>(sql2);
Does anyone have a relatively painless way of doing this?
Like Fredrik says, the value from SqlDataReader is boxed. You can convert a boxed value to an int with Convert.ToInt32, like:
int i = Convert.ToInt32(read[0]);
This will try to convert even if SQL Server returns a bigint or a decimal.
System.Convert will take care of the conversion.
T GetValue<T>(SqlDataReader rdr)
{
var dbVal = rdr.GetValue(0);
var csVal = (T)System.Convert.ChangeType(dbVal, typeof(T));
}
Caveat: if T == Nullable<S>, you need to do some extra work with reflection to get the underlying type and call ChangeType with typeof(S) as the type parameter. Apparently, MS didn't update the ChangeType function with .NET 2.0 and the introduction of nullables. And if it's a nullable, and dbVal is DBNull, you can just return null.
object dbValue = 5;
//this throws
Convert.ChangeType(dbValue, typeof(int?));
//this works
if(dbValue == DBNull.Value || dbValue == null)
{
if(typeof(int?).IsNullable) //or is a class, like string
{return null;}
dbValue = null;
}
var type = GetUnderlyingType<int?>(); //== typeof(int)
Convert.ChangeType(dbValue, type);
I think your problem is that GetValue returns an object. This means that in the case of an int, you will get an int boxed in an object. Then you cannot directly cast it to a long but must first unpack it as an int:
var myLong = (long)(int)rdr.GetValue(0);
This will be quite tricky using generics, I would say. Well, you could make generic methods with two type arguments; one specifying what type the field is, and one specifying the type you want. But I don't really see the need; SqlDataReader already has methods for the various data types, such as GetInt32, GetInt64 and so on, so the generic method would not really give any added value in that case.