c# - Using code contracts in one property to provide hints about another property -

i have struct (simplified brevity):

public struct period {     public datetime? start { get; private set; }     public datetime? end { get; private set; }      public bool ismoment     {         { return this.start.hasvalue && this.start == this.end; }     }      public period(datetime? start, datetime? end) : this()     {         this.start = start;         this.end = end;     }      public override string tostring()     {         return this.ismoment              ? this.start.value.tostring("g")              : string.format("{0:g} – {1:g}", this.start, this.end);     } } 

everything works fine, resharper showing warning on this.start.value.tostring:

possible 'system.invalidoperationexception'

if copy body of ismoment property condition, warning goes away, i'd able reuse property. can disable resharper warning comment (which have done moment), or changing tostring string.format, have handful of other places in code , got me thinking. i'd try resolve using code contracts, unfortunately don't have whole lot of experience code contracts , i'm not sure how look.

is there anyway use code contracts indicate resharper if ismoment true, start not null?

do in ismoment:

contract.ensures(result == false || start <> null);

(that's not right. i'm typing on phone.)

update:

the problem due possibility of multithreaded code changing value of start between ismoment test , start.value evaluation.

it may more correct copy value local variable cannot modified other threads.

public override string tostring() {     period local = this;     return local.ismoment          ? local.start.value.tostring("g")          : string.format("{0:g} – {1:g}", local.start, local.end); } 

this looks unnecessary work, , may inefficient, more "correct". if struct small, though, may more efficient in many cases.