Overriding code-generated DbContext constructor

Question

I'm sure I've done this before at some stage, but I can't figure out how to now! My scenario:

// This is generated from EDMX
public partial class HOLDbEntities : DbContext
{
    public HOLDbEntities()
            : base("name=HOLDbEntities")
        {
        }
}

Now, I want this connection string to be easily changeable (I want to Implement from the HOLDbEntities), so I need to override this constructor.

I've tried:

public partial class HOLDbEntities
{
    private const string _contextName = "HOLDbEntities";
    public static string ContextName { get { return _contextName; } }

    public HOLDbEntities()
        : base(ContextName)
    {
    }
}

But this throw an error:

HOLDbEntities already defines a member called "HOLDbEntities" with the same parameter types.

I can understand why this errors, but how would I stop the constructor being auto-generated in the first place in order to do what I'm trying to achieve?

Solution 1

The best I can suggest is a factory method:

private HOLDbEntities(string contextName) : base(contextName) { }

public static HOLDbEntities Create() {
    return new HOLDbEntities(ContextName);
}

and use HOLDbEntities.Create() rather than new HOLDbEntities().

Solution 2

I up-voted the previous accepted answer because it is a fairly elegant way of doing it. However another approach would be to modify the T4 template that generates the dbContext Class.

When using EF DB first you have a .edmx file and under that you have an [Entity].Context.tt file. Go into that file and remove (or modify) the following code:

public <#=code.Escape(container)#>()
        : base("name=<#=container.Name#>")
    {
<#
if (!loader.IsLazyLoadingEnabled(container))
{
#>
        this.Configuration.LazyLoadingEnabled = false;
<#
}

foreach (var entitySet in container.BaseEntitySets.OfType<EntitySet>())
{
    // Note: the DbSet members are defined below such that the getter and
    // setter always have the same accessibility as the DbSet definition
    if (Accessibility.ForReadOnlyProperty(entitySet) != "public")
    {
#>
        <#=codeStringGenerator.DbSetInitializer(entitySet)#>
<#
    }
}
#>

now your context class will generate without a constructor, so you should be able to go and create one in an extended class.

Solution 3

i changed the context.tt as follows:

<#=Accessibility.ForType(container)#> partial class <#=code.Escape(container)#> : DbContext
{
    public <#=code.Escape(container)#>()
        : base("name=<#=container.Name#>")
    {

<#
if (!loader.IsLazyLoadingEnabled(container))
{
#>
        this.Configuration.LazyLoadingEnabled = false;
<#
}
foreach (var entitySet in container.BaseEntitySets.OfType<EntitySet>())
{
    // Note: the DbSet members are defined below such that the getter and
    // setter always have the same accessibility as the DbSet definition
    if (Accessibility.ForReadOnlyProperty(entitySet) != "public")
    {
#>
        <#=codeStringGenerator.DbSetInitializer(entitySet)#>
<#
    }
}
#>
var Method = (typeof(Entities)).GetMethods(System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance).FirstOrDefault(x => x.Name == "OnModelConstructed");
if (Method!=null) Method.Invoke(this,null);
    }

so i can declare a OnModelConstructed method in a partial class of the context.

Comments

[Chris Dixon]

This seems very elegant to what I'm trying to achieve, I'll give it a shot now and see if it works!

[Chris Dixon]

Works fine, but Unity is now complaining that the context cannot be constructed with a String value. Any way around this? Your answer is correct for my scenario anyway.

[alamin]

in MSDN it says DbContext constructor overriding with a string Constructs a new context instance using the given string as the name or connection string for the database to which a connection will be made. - can you please you explain it to me clearly or give me an example or reference. i did not find any good result while searching for it :(

[as9876]

Won't the T4 code also get regenerated if you, say, drag another table onto the designer?

[Dylan Hayes]

@AvrahamSeff nope, the t4s stay the same, and generate .cs files. any changes to the .cs files would get overrwitten in the way that you describe, but this is a change to the file that is doing the generating.

[Admin]

@as9876 the T4 code includes generic code that iterates over the entity types, so it doesn't need to be regenerated when new entity types are added

[Jonathan Wood]

If you're going to edit the T4 template, then why not just modify the constructor there instead of removing it and adding another partial class?