惠威订单导入，判断字段哪个超长？

Question

I've created an EDMX object from a database I'm programming against.

I need to get input from a user and save it to a row in the database table. The problem is that I need to limit the length of input strings to the width of the corresponding VARCHAR column in the database.

When I browse the model, I can clearly see in the properties window that the model knows the max length of the string, but I don't know how to access this data in code.

If I want to write something like this:

Entities entities = new Entities();
myTable = entities.myTable.First();
if (userInput.length > myTable.columnA.MaxLength)
{
    // tell the user that the input is too long.
}
else
{
    myTable.columnA = userInput;
}

How do I write it?

Update: I would like to point out that the IObjectContextAdapater mentioned in the answers below is in the System.Data.Entity.Infrastructure namespace.

I'm surprised by the low number of views on this question. Thanks very much for the helpful answers; I might have never figured it out on my own. – Daniel Allen Langdon Nov 20 '14 at 20:35 — Daniel Allen Langdon, Nov 20 '14 at 20:35

Gert Arnold · Accepted Answer · 2014-11-19 19:29:07Z

Here are two methods by which you can read the meta data:

int? GetMaxLength(DbContext context, string tableName, string propertyName)
{
    var oc = ((IObjectContextAdapter)context).ObjectContext;

    return oc.MetadataWorkspace.GetItems(DataSpace.CSpace).OfType<EntityType>()
             .Where(et => et.Name == tableName)
             .SelectMany(et => et.Properties.Where(p => p.Name == propertyName))
             .Select (p => p.MaxLength)
             .FirstOrDefault();
}

int? GetMaxLength<T>(DbContext context, Expression<Func<T, object>> property)
{
    var memberExpression = (MemberExpression)property.Body;
    string propertyName = memberExpression.Member.Name;
    return GetMaxLength(context, typeof(T).Name, propertyName);
}

So you can either enter the table name and property name, or an expression that specifies the property you're interested in.

Another approach could be to create a MetaData class and use the MaxLength attribute.

I'm confused. I thought that the EDMX entities is an ObjectContext, which has nothing to do with DbContext. – Daniel Allen Langdon Nov 19 '14 at 22:19 — Daniel Allen Langdon, Nov 19 '14 at 22:19
Not necessarily. You'd have to check. If it is an ObjectContext you don't need this ((IObjectContextAdapter)context).ObjectContext part. – Gert Arnold Nov 19 '14 at 22:21 — Gert Arnold, Nov 19 '14 at 22:21
I did check and I see that it's a DbContext. To be honest, I have heard of DbContext, but I haven't read up on all the new features of the new Entity Framework versions, so this one got past me. Wow, there is so much keeping up to do in the .net world! – Daniel Allen Langdon Nov 19 '14 at 22:24 — Daniel Allen Langdon, Nov 19 '14 at 22:24
The function GetItems returns an enumeration of 393 items. However, the OfType<myTable> function, where myTable is a model of a SQL table, returns an empty enumeration. – Daniel Allen Langdon Nov 19 '14 at 22:56 — Daniel Allen Langdon, Nov 19 '14 at 22:56
You should leave EntityType there, it's an EF class. – Gert Arnold Nov 19 '14 at 23:05 — Gert Arnold, Nov 19 '14 at 23:05

score 1down vote · Answer 2

It's not very pretty; reading edmx properties at runtime is not something Microsoft exposed easily or documented well (or in some cases, at all). context is your DBContext.

var objectContext = ((IObjectContextAdapter)context).ObjectContext;
var entityType = objectContext.MetadataWorkspace.GetItems<EntityType>(DataSpace.CSpace).Where(e => e.Name == "your entity name").First();
var facets = entityType.Properties["your property name"].TypeUsage.Facets;

facets will look something like this, so you'll need to look for the MaxLength Name(may not exist, depending on the underlying field type) and get the Value:

Count = 5
    [0]: Nullable=false
    [1]: DefaultValue=null
    [2]: MaxLength=250
    [3]: Unicode=false
    [4]: FixedLength=false

不及格的程序员-八神

惠威订单导入，判断字段哪个超长？

How to get the maximum length of a string from an EDMX model in code?

2 Answers

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