C#: How to set default value for a property in a partial class?

I'm very new to C# so please bear with me...

I'm implementing a partial class, and would like to add two properties like so:

public partial class SomeModel
{
    public bool IsSomething { get; set; }
    public List<string> SomeList { get; set; }

    ... Additional methods using the above data members ...
}

I would like to initialize both data members: IsSomething to True and SomeList to new List<string>(). Normally I would do it in a constructor, however because it's a partial class I don't want to touch the constructor (should I?).

What's the best way to achieve this?

Thanks

PS I'm working in ASP.NET MVC, adding functionality to a a certain model, hence the partial class.

Solution

Updated for C# 6

C# 6 has added the ability to assign a default value to auto-properties. The value can be any expression (it doesn't have to be a constant). Here's a few examples:

// Initialize to a string literal
public string SomeProperty {get;set;} = "This is the default value";

// Initialize with a simple expression
public DateTime ConstructedAt {get;} = DateTime.Now;

// Initialize with a conditional expression
public bool IsFoo { get; } = SomeClass.SomeProperty ? true : false;

Original Answer

Automatically implemented properties can be initialized in the class constructor, but not on the propery itself.

public SomeModel
{
    IsSomething = false;
    SomeList = new List<string>();
}

...or you can use a field-backed property (slightly more work) and initialize the field itself...

private bool _IsSomething = false;
public bool IsSomething
{
    get { return _IsSomething; }
    set { _IsSomething = value; }
}

Update: My above answer doesn't clarify the issue of this being in a partial class. Mehrdad's answer offers the solution of using a partial method, which is in line with my first suggestion. My second suggestion of using non-automatically implemented properties (manually implemented properties?) will work for this situation.