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Entity Framework: One model for multiple tables with non identical structures

I have more than 50 data tables that have nearly identical structures. Some of the tables have additional columns. I'm developing an application to help me monitor and track changes to the data contained in these tables and only need to be able to read the data contained in them. I want to create an entity framework model that will work with all of the tables and give me access to all columns that exist.
As long as the model contains the subset of columns that exist is all of the tables my model works and I can dynamically switch between the tables with the same model. However I need accesses to the additional columns when they exist. When my model contains a column that doesn't exist in the table that I switch to I get an exception for an invalid column. Is there a way to have my model be the set of all columns and if the column doesn't exist in the context of a particular table handle it in a way that I still have access to the columns that exist? I know that using strait SQL I can do this quite easily but I'm curious is there is a way to do this with entity framework. Essentially I am looking for the equivalent of querying sys.columns to determine the structure of the table and then interact with the table based on knowing what columns exist from the sys.columns query.
Sample of issue:
The 50+ tables hold data from different counties. Some of there counties have included additional data, for instance a url link to an image or file. Thus I have an column that is a varchar that contains this link. Many of the counties don't supply this type of attribute and it isn't apart of the table in other counties. But there are 100 other reported attributes that are common between all tables. I realize a solution to this issue is to have all tables contain all possible columns. However in practice this has been hard to achieve due to frequent changes to provide more to our clients in certain counties.

From the EF prospective I do not know a solution but you can try something with an extension method like below:
public static DbRawSqlQuery<YourBaseModel> GetDataFromTable(this ApplicationDbContext context, string tableName)
{
return context.Database.SqlQuery<YourBaseModel>("select * from " + tableName);
}
I think this will map only columns that exists in table with properties in your model.
This is not tested by the way but it can give you an idea of what I mean.

Entity Framework supports generating Table per Concrete type mapping, this lets you have a base class that contains all the shared columns, and derived classes for each specific table
https://weblogs.asp.net/manavi/inheritance-mapping-strategies-with-entity-framework-code-first-ctp5-part-3-table-per-concrete-type-tpc-and-choosing-strategy-guidelines

How can I denormalize my clr entity, but keep my db tables normalized?

I have these two related tables Client (ClientId, Name) and ClientDescription (ClientDescriptionId, (FK) ClientId, Description). That is to say each Client can have many associated descriptions. Now, when displaying the a list of ClientDescriptions, I also need to know what the Name of it's associated Client is.
Now you'll probably say that I allready have this information, since I can simply follow my navigation property back to the associated Client and use its Name. I can't do that because I'm autogenerating a grid in Ria services, and this just gives me a count for navigation properties, and I haven't found a way to flatten this down in my metadata file. Hence why I want a property.
The whole idea is that I want to be able to add a new field to my database, update my entity classes from the database and regenerate my domain service, and the new field should just pop up in my grid. I shouldn't have to update my xaml just because my database happen to have an extra field.
So, what I would like to do is add a ClientName field to the entity (clr object), but keep my database clean (no such denormalization in the db).
So, I generated my edmx, and added a new property named ClientName. Set it to StoreGeneratedPattern.Computed, and compiled. I then get a nasty little error
Error 3004: Problem in mapping fragments starting at line NN: No mapping specified for properties (etc..)
The solution apparently is to generate my database from my edmx. (Or that's what answers to questions about that error seems to yield.) But this generates an actual DB-field, which I don't want, so that answer doesn't apply to my case.
So my question is: How can I denormalize my clr entity, but keep my db tables normalized?
Edit: I guess this question can be generalized a bit. The issue would be the same if ClientDescription contained a few numeric fields that I wanted to do some calculations on, and I wanted the result available as a field and the algorithm should be in c# rather than in my database.

To answer your more generalized question:
Entities are generated by the Entity Framework with a partial keyword.
This means that the code of an entity can be split in multiple source files in the same namespace and assembly. One will contain the generated code from the Entity Framework, the other will contain custom properties and methods.
If for example, your entity has the database fields Price and Amount you could add a property in the partial class TotalPrice which would return Price * Amount.
Then the algorithm will be C# and your database won't know about the extra property.

Entity Framework Design - Multiple "Views" for the data

I have a design question related to Entity Framework entities.
I have created the following entity:
public class SomeEntity {
// full review details here
}
This entity has as an example 30 columns. When I need to create a new entity this works great. I have all of the required fields in order to insert into the database.
I have a few places in my app where I need to display some tabular data with some of the fields from SomeEntity, but I don't need all 30 columns, maybe only 2 or 3 columns.
Do I create an entirely new entity that has only the fields I need (which maps to the same table as SomeEntity, but only retrieves the column I want?)
Or does it make more sense to create a domain class (like PartialEntity) and write a query like this:
var partialObjects = from e in db.SomeEntities
select new PartialEntity { Column1 = e.Column1, Column2 = e.Column2 };
I am not sure what the appropriate way to do this type of thing. Is it a bad idea to have two entities that map to the same table/columns? I would never actually need the ability to create a PartialEntity and save it to the database, because it wouldn't have all of the fields that are required.

Your first approach is not possible. EF doesn't support multiple entities mapped to the same table (except some special cases like TPH inheritance or table splitting).
The second case is common scenario. You will create view model for your UI and either project your entity to view model directly in query (it will pass from DB only columns you project) or you will query whole entity and make conversion to view model in your application code (for example by AutoMapper as #Fernando mentioned).
If you are using EDMX file for mapping (I guess you don't because you mentioned ef-code-first) you can use third approach which takes part from both mentioned approaches. That approach defines QueryView - it is EF based view on the mapped entity which behaves as a new read only entity. Generally it is reusable projection stored directly in mapping.

What you proposed as a first solution is the "View model paradigm", where you create a class for the sole purpose of being the model of a view to retrieve data and then map it to the model class. You can use AutoMapper to map the values. Here's an article on how to apply this.

You could create a generic property filter method that takes in an object instance, and you pass in a string array of column names, and this method would return a dynamic object with only the columns you want.

I think it would add unnecessary complexity to your model to add a second entity based on the same data structure. I honestly don't see the problem in having a single entity for updating\editing\viewing. If you insist on separating the access to SomeEntity, you could have a database view: i.e. SomeEntityView, and create a separate entity based on that.

Entity Framework: Mapping sproc results to existing EF Entity

I am trying to use EF with an existing DB. I brought in a Client table into my data model and let EF create a Client entity. I have a sproc, GetClientSearch, that only returns 5 out of the 15 columns from the Client table becuase that is all that is needed for that call.
Here's what I've done so far:
Added the sproc to Function Imports and set the proc to map to the Client entity.
When I execute the proc through the Context, I get "The data reader is incompatible with the specified 'GAINABSModel.Client'. A member of the type, 'MiddleInitial', does not have a corresponding column in the data reader with the same name." exception. (MiddleInitial is not one of the columns returned in the proc)
I know that I can create a new entity that maps to the proc, but I don't want to do that for every proc I have to import into my model.
Given that the DB is currently in use in production, changing stored procs to map to my current entities may not be an option.
Currently using EF 4 and VS 2010.
So, is there a way to map the results of the sproc to the Client entity, even though the columns returned are not 1:1 with the properties of the EF entity?

Yep, one of my many pain points in EF.
If you can't modify the SP's, your best bet might be to create "wrapper" SP's on top of the existing SP's.
In other words, EF-serving SP's that call into the existing ones, and return NULL for the columns you don't need, but are required for the entity.
Of course the better option would be to create the entities properly.
Another option is to use ObjectContext.Translate<T>, which basically performs a L-R between the SPROC results and the entity you supply.
If the result set doesn't contain the field, then the property on the object will be null.
Which is probably what you want.

Am running into the same Issues. Suppose i have UserEntity created out of the User Table and have 3 procedures.
AuthenticateUser - returns 4 columns from the user table after authentication
RetriveUser - Returns 10 columns from the user table
GetUserName - return UserID and UserName only for dropdown purpose.
If we create different entities for each of the different SP. It would result in bad design because of duplication.
I have no other way of using same entity for all these SP's.
Overall, i don't recommend entity framework atleast for legacy applications in production.(where you can not update your Sp's also.)

NHibernate mapping class properties to Rows not Columns

Is it possible to map a class with each property stored as a row in the table, not a column. The scenario is where we persist global options to the database. We store the options in an 'Options' class that has a property per option, i.e. "Expand Menu", "Save on Exit" etc.
Rather than store each option in its own table column, we would simply like to have a table with each of the class properties stored as a new row, identified by a Enum.
Is this possible?
(C# Winforms)

Using NHibernate's EntityMode.Map might help solve this problem. It does mean that you might have to put your global options in a Dictionary collection, but you can always implement an Option class that abstracts the underlying Dictionary.
NHibernate provides the ability to map a Dictionary dynamically to a table. See NHibernate's reference on Dynamic Models