How can I make one query out of many others

User 11883281

I've this code: (it's much longer...)

if (selection == "bild" )
{
erg = from item in _db.object_1
where item.article_number != null
&& item.bild == null
&& item.brand.StartsWith(marke)
&& ( item.o_path.StartsWith(pathwerkzeug) || item.o_path.StartsWith(pathgarten))
select item.article_number;
}
else if (selection == "machine_group")
{
erg = from item in _db.object_1
where item.article_number != null
&& item.machine_group == null
&& item.brand.StartsWith(marke)
&& (item.o_path.StartsWith(pathwerkzeug) || item.o_path.StartsWith(pathgarten))
select item.article_number;
}

How can I shorten it? Can I get a Variable for for example "item.bild"? Its always the same... if the selection is "bild" I will ask if "item.bild" is null; if the selection is "machine_group" I will ask if "item.machine_group" is null Can you help me?

Richard Deeming

You can extract the common part of the query, and apply the additional filters to that common part:

var query = from item in _db.object_1
where item.article_number != null
&& item.brand.StartsWith(marke)
&& (item.o_path.StartsWith(pathwerkzeug) || item.o_path.StartsWith(pathgarten));

if (selection == "bild" || selection == "Alle")
{
erg = from item in query
where item.bild == null
select item.article_number;
}
else if (selection == "machine_group") // || selection == "Alle")
{
erg = from item in query
where item.machine_group == null
select item.article_number;
}

NB: There's no point having selection == "Alle" in your else if test. If selection == "Alle", your code will execute the if branch, and ignore the else if branch. If you've got a lot of branches, you should probably look at using a switch block:

var query = from item in _db.object_1
where item.article_number != null
&& item.brand.StartsWith(marke)
&& (item.o_path.StartsWith(pathwerkzeug) || item.o_path.StartsWith(pathgarten));

switch (selection)
{
case "bild":
case "Alle":
{
erg = from item in query
where item.bild == null
select item.article_number;
break;
}
case "machine_group":
{
erg = from item in query
where item.machine_group == null
select item.article_number;
break;
}
}

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

User 11883281

I know.. the "else if" have to be an "if" please ignor it Thank you thats much shorter... but maybe its going shorter?

Pete OHanlon

Why not use the Dynamic LINQ Library to do this? Here's some useful info: ScottGu[^].

User 11883281

Thank you, but I do not have experience in dynamic Linq, and i don't have time to study it now, isn't there any other way?

Pete OHanlon

No, there isn't and Dynamic LINQ isn't hard - you could have learned it in the time you waited for my reply. Honestly, it looks like you didn't even read the link I put in the previous post - if you had, you'd have seen that you can mix and match your standard "from x in ...." syntax with Dynamic LINQ, so you only need to touch the one place.

Richard Deeming

Not sure it's technically shorter, but you could build a dictionary to map your selection variable to a filter, and store that map as a private field in your class:

private static readonly Dictionary<string, Expression<Func<object_1, bool>>> Filters = new Dictionary<string, Expression<Func<object_1, bool>>>
{
{ "bild", item => item.bild == null },
{ "machine_group", item => item.machine_group == null },
};

You would then use that dictionary to look up the filter:

var query = from item in _db.object_1
where item.article_number != null
&& item.brand.StartsWith(marke)
&& (item.o_path.StartsWith(pathwerkzeug) || item.o_path.StartsWith(pathgarten));

Expression<Func<object_1, bool>> filter;
if (Filters.TryGetValue(selection, out filter))
{
query = query.Where(filter);
}

erg = query.Select(item => item.article_number);

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

User 11883281

Thank you that looks good, i will try it.

User 11883281

Okay now I get a few errors... 1.:

A query body must end with a select clause or a group clause

2.:

'System.Collections.Generic.IEnumerable' does not contain a definition for 'Where' and the best extension method overload 'System.Linq.Queryable.Where(System.Linq.IQueryable, System.Linq.Expressions.Expression>)' has some invalid arguments

3.:

The type of the expression in the select clause is incorrect. Type inference failed in the call to 'Select'.

4.:

The type arguments for method 'System.Linq.Enumerable.Select(System.Collections.Generic.IEnumerable, System.Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

5.:

Instance argument: cannot convert from 'System.Collections.Generic.IEnumerable' to 'System.Linq.IQueryable'

Richard Deeming

I suspect they're all related. Try adding a select clause to the end of the first query:

var query = from item in _db.object_1
where item.article_number != null
&& item.brand.StartsWith(marke)
&& (item.o_path.StartsWith(pathwerkzeug) || item.o_path.StartsWith(pathgarten))
select item;

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer

User 11883281

Thanks ;) it WAS working ... and then I changed it for all and now theres this errro:

System.TypeInitializationException: Der Typeninitialisierer für "Quälix.AbfrageIsNull" hat eine Ausnahme verursacht. ---> System.ArgumentException: Ein Element mit dem gleichen Schlüssel wurde bereits hinzugefügt.
bei System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
bei System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
bei System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
bei Quälix.AbfrageIsNull..cctor() in q:\PIM_PolicingTool\Quälix\Quälix\AbfrageIsNull.cs:Zeile 12.
--- Ende der internen Ausnahmestapelüberwachung ---
bei Quälix.AbfrageIsNull.ArtikelAufNullPruefen(String selection, List`1 marken)
bei Quälix.Form1.btAbfragIsNullSuche_Click(Object sender, EventArgs e) in q:\PIM_PolicingTool\Quälix\Quälix\Form1.cs:Zeile 56.

this is my code:

if (chbAbfrageArtikel.Checked == true)
{
endErgebnis += "ARTIKEL:";
endErgebnis += Environment.NewLine;
string zwischenErg = AbfrageIsNull.ArtikelAufNullPruefen(selection, marken);

                if (zwischenErg == "") { endErgebnis += "Alles zugeordnet"; }
                else { endErgebnis += zwischenErg; }

                endErgebnis += Environment.NewLine;
                endErgebnis += Environment.NewLine;
            }
            if (chbAbfrageZubehoer.Checked == true)
            {
                endErgebnis += "ZUBEHÖR:";
                endErgebnis += Environment.NewLine;
                string zwischenErg =  "";// AbfrageIsNull.ZubehoerAufNullPruefen(selection, marken);

                if (zwischenErg == "") { endErgebnis += "Alles zugeordnet"; }
                else { endErgebnis += zwischenErg; }

                endErgebnis += Environment.NewLine;
                endErgebnis += Environment.NewLine;
            }
            if (chbAbfrageErsatzteil.Checked == true)
            {
                endErgebnis += "ERSATZTEILE:";
                endErgebnis += Environment.NewLine;
                string zwischenErg = "";// AbfrageIsNull.ErsatzteilAufNullPruefen(selection, marken);                    
                if (zwischenErg == "") { endErgebnis += "Alles zugeordnet"; }
                else { endErgebnis += zwischenErg; }

Richard Deeming

The error message is quite clear - "An item with the same key has already been added". In your dictionary initializer, you have added the key "Inventurart" twice - the 6th and 29th items. Remove one of those items.

"These people looked deep within my soul and assigned me a number based on the order in which I joined." - Homer