c# - Code contracts, forall and custom enumerable -

i using c# 4.0 , code contracts , have own custom gameroomcollection : ienumerable<gameroom>.

i want ensure, no instances of gameroomcollection ever contain null value element. don't seem able this, though. instead of making general rule, have tried plain , simple example. allgamerooms instance of gameroomcollection.

private void setuplisteners(gameroom newgameroom) {   contract.requires(newgameroom != null);   //... } private void setuplisteners(model model) {     contract.requires(model != null);     contract.requires(model.allgamerooms != null);     contract.assume(contract.forall(model.allgamerooms, g => g != null));     foreach (gameroom gameroom in model.allgamerooms)         setuplisteners(gameroom);//<= warning: code contracts: requires unproven: newgameroom != null  } 

can see, why haven't proven, gameroom not null?

edit:

adding reference object before iterating not work either:

ienumerable<igameroom> gamerooms = model.allgamerooms; contract.assume(contract.forall(gamerooms, g => g != null)); foreach (igameroom gameroom in gamerooms)     setuplisteners(gameroom);//<= warning: code contracts: requires unproven: newgameroom != null  

edit2:

however: if convert game room collection type array, works fine:

igameroom[] gameroomarray = model.allgamerooms.toarray(); contract.assume(contract.forall(gameroomarray, g => g != null)); foreach (igameroom gameroom in gameroomarray)     setuplisteners(gameroom);//<= no warning 

is caused fact, cannot define rule methods of ienumerable<t> interface?

edit3: can problem somehow related this question?

i think might have purity of getenumerator method. pureattribute

contracts accept methods defined [pure] (side effect free).

some information code contracts, purity

qoute:

purity

all methods called within contract must pure; is, must not update preexisting state. pure method allowed modify objects have been created after entry pure method.

code contract tools assume following code elements pure:

methods marked pureattribute.

types marked pureattribute (the attribute applies type's methods).

property accessors.

operators (static methods names start "op", , have 1 or 2 parameters , non-void return type).

any method qualified name begins "system.diagnostics.contracts.contract", "system.string", "system.io.path", or "system.type".

any invoked delegate, provided delegate type attributed pureattribute. delegate types system.predicate , system.comparison considered pure.