var keyword

CONSIDER using the var keyword whenever the resulting type of the variable is obvious. Avoid using it if it decreases readability.

// Good usage: Type is obvious from the right side
var names = new List<string>();

// Explicit type needed if GetSource() return type isn't obvious
string source = GetSource();

// OK: String.Split return type (string[]) is well-known
var tokens = source.Split(' ');

// Probably not good: Return type of GetId() is unclear
var id = GetId();

Framework Design Guidelines Conventions, Idioms, and Patterns for Reusable .NET Libraries Second Edition (367)

Official Microsoft recommendation

var requires less typing. It also is shorter and easier to read.
var encourages you to use a descriptive name for the variable.
You shouldn't have to care what the type of the object is
TBD

Add more information.

In this example:

// Explicitly typed as IEnumerable - might fetch all data
IEnumerable<CaseTable> test = from x in dataBase.CaseTables
                              where x.case_id.Equals("12")
                              select x;

If you wrote test.First() you could retrieve lots of rows from the database to get only the first one. If you used var or explicitly IQueryable<CaseTable>, you would retrieve only the first row (assuming dataBase.CaseTables is IQueryable).

If the underlying type returned by the query is actually IQueryable<CaseTable> but you assign it to IEnumerable<CaseTable>, then the performance when calling methods like First() could be much slower (fetching all data first) although the code compiles and looks correct. Using var here would correctly infer IQueryable<CaseTable> if that's what the LINQ provider returns.