Develop Reference

How do I select all distinct strings in a list of lists of another type?

I'm still very new with LINQ. I have the following "simplified" data structure:
List<List<Field>> myData = new List<List<Field>>();
Field consists of two string members, Type and Name.
My goal is to get a List<string> containing all distinct Name corresponding to a given Type. My first approach is this:
var test = myData
.Where(a => a.FindAll(b => b.Type.Equals("testType"))
.Select(c => c.Name)
.Distinct());
Does somebody have a hint for me? =)

You just need to use SelectMany to flatten your list of lists and then proceed as normal
var test = myData.SelectMany(x => x)
.Where(x => x.Type == "testType")
.Select(x => x.Name)
.Distinct()
.ToList();
Or in query syntax
var test = (from subList in myData
from item in subList
where item.Type == "testType"
select item.Name).Distinct().ToList();

Another way to do it using query notation:
var test= from list in myData
from e in list
where e.Type=="testType"
group e.Name by e.Name into g
select g.Key;
But is better go for one of the #juharr's solutions

Linq query to select all numbers that match

I have some difficulties to query a table. In the table there are a column for ResourceID and a column for ProjectID. I want to get a list of all int numbers from the ResourceID where ProjectID match the projId number. Is this possible?
I have tested, but I guess this is wrong. I thought I could get the value by using the resources variable, but that isn't working.
var resources = db.Activities.Where(x => x.ProjectID == projId).ToList();
resources. ????

You could accomplish this via a Select() query to only pull the specific properties that you need (in this case the ResourceID property) for each of the elements within your collection :
// Get your resources that meet your requirement
var resources = db.Activities.Where(x => x.ProjectID == projId).ToList();
// Get your Resource IDs from your previous query
var resourceIds = resources.Select(r => r.ResourceID);
You could actually just perform this in a single call if you preferred :
// This will return a list of ResourceIDs that match
var resourceIds = db.Activities.Where(a => a.ProjectID == projId)
.Select(a => a.ResourceID)
.ToList();

I believe the answer is:
var resources = db.Activities.Where(x => x.ProjectID == projId).Select(x => x.ResourceID).ToList();

You're getting a list of the Activities. What you need to do is to select the item you want--in this case the ResourceID column.
var resources = db.Activities.Where(x => x.ProjectID == projId)
.Select(x => x.ResourceID)
.Distinct() // Thought you might want this
.ToList();
You can also use a different format like this, which can sometimes be clearer.
var resources = (from a in db.Activities
where a.ProjectID == projId
select a.ResourceID).Distinct().ToList();

Please update your query if you want to get list of int..
var resources = db.Activities.Where(x => x.ProjectID == projId).Select(x => x.ResourceID).ToList();

Linq IEnumerable<IGrouping<string, Class>> back to List<Class>

How can I turn the following statement back to List<DocumentData>
IEnumerable<IGrouping<string, DocumentData>> documents =
documentCollection.Select(d => d).GroupBy(g => g.FileName);
the goal is to get List that should be smaller than documentCollection.
FileName contains duplicates so I want to make sure I don't have duplicate names.
I have also tried the following but it's still providing me with duplicate file names
documentCollection =
documentCollection.GroupBy(g => g.FileName).SelectMany(d => d).ToList();

Each IGrouping<string, DocumentData> is an IEnumerable<DocumentData>, so you could simply call SelectMany to flatten the sequences:
var list = documents.SelectMany(d => d).ToList();
Edit: Per the updated question, it seems like the OP wants to select just the first document for any given filename. This can be achieved by calling First() on each IGrouping<string, DocumentData> instance:
IEnumerable<DocumentData> documents =
documentCollection.GroupBy(g => g.FileName, StringComparer.OrdinalIgnoreCase)
.Select(g => g.First())
.ToList();

You haven't said what T should stand for in List<T> you're looking for, so here are couple the most likely to be desired:
List<DocumentData> - rather pointless as you already have that on documentCollection
var results = documents.SelectMany(g => g).ToList();
List<KeyValuePair<string, List<DocumentData>>
var results =
documents.Select(g => new KeyValuePair(g.Key, g.ToList())).ToList();
List<string> - just the names
var results = documents.Select(g => g.Key).ToList();
List<IGrouping<string, DocumentData>>
var results = documents.ToList();

C# XML Fetching

My XML looks so:
<sensor>
<data>26.7</data>
<data>54.53</data>
<log>false</log>
</sensor>
To retrieve the data fields in a list, I use:
List<string> list = xml.Root.Descendants("sensor").Elements("data").Select(element => element.Value).ToList();
And it works well.
Sometimes the XML looks like this (log = true):
<sensor>
<data>26.7</data>
<data>54.53</data>
<log>true</log>
</sensor>
And I want to ignore these values. How can I achieve this?
I tried this:
var lastUser = xml.Element("response").Descendants("sensor")
.First(u => u.Element("data") != null
&& u.Element("log").Value == "false");
But I only can retrieve of course only the first value.

Why do you use First if you don't want to select the first element? Use Where instead.
var data = xml.Element("response")
.Descendants("sensor")
.Where(x => ((string)x.Element("log")) == "false")
.Elements("data")
.Select(x => x.Value)
.ToList();

Why not use .Where instead of .First and you should be getting an array back?

Use casting nodes to bool or to double:
var data= xml.Element("response").Descendants("sensor")
.Where(s => (bool)s.Element("log"))
.Elements("data")
.Select(d => (double)d)
.ToList(); // produces List<double>
And yes #Daniel is right - First returns only first element from sequence which matches your conditions.
Also consider to use XPath
var data = xdoc.XPathSelectElements("response/sensor[log='true']/data")
.Select(d => (double)d)
.ToList();

Return nested alias for linq expression

I have the following Linq Expression
var tooDeep = shoppers
.Where(x => x.Cart.CartSuppliers.First().Name == "Supplier1")
.ToList();
I need to turn the name part into the following string.
x.Cart.CartSuppliers.Name
As part of this I turned the Expression into a string and then split on the . and removed the First() argument. However, when I get to CartSuppliers this returns a Suppliers[] array. Is there a way to get the single type from this. eg. I need to get a Supplier back.
Update: Got it to work
var fullName = m.ToString().Split(new[] { '.' }, StringSplitOptions.RemoveEmptyEntries);
// this supports the "deep graph" name - "Product.Address.City"
var fixedName = fullName.Skip(1).Take(fullName.Length - 1)
.Where(x => x != "First()")
.Select(x => System.Text.RegularExpressions.Regex.Replace(x, #"\[[0-9]+\]",""))
.ToArray();
with this:
var prop = property.PropertyType.HasElementType ? property.PropertyType.GetElementType() property.PropertyType;
which enabled be to find the individual type from an array.
Thanks

firstSupplierWithNeededName = shoppers
.SelectMany(s => s.Cart.CartSuppliers)
.First(s => s.Name == "Supplier1");
But also look into using FirstOrDefault or Single if it has to return just one.