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LINQ query needed using a conditional operand in the WHERE clause

I am having trouble composing a LINQ statement that uses a conditional operand within the where clause and have looked through this site for a similar problem without any luck. I'm sure there is a better way to express this query. The value XXX within the is the result of determining which field in the object to use for comparison and is explained below.
Below is what I am trying to accomplish:
var paymentReceivedAmt = (from registerEntry in RegisterEntries
where registerEntry.TransactionType != null
&& registerEntry.TransactionType.Id.SubsystemCode == "A"
&& registerEntry.Receipt != null
&& registerEntry.PostDate != null
&& XXX >= lastInvoicedDate
select registerEntry.TransactionAmount.GetValueOrDefault(0)).Sum();
Value XXX =
if registerEntry.AddedDate = registerEntry.PostDate
registerEntry.AddedDate
else
registerEntry.PostDate
The values registerEntry.AddedDate and registerEntry.PostDate are DateTime type, however when comparing I need to only compare the date and not the time
Does anyone have any idea on how to do this?

Why not just use registerEntry.PostDate for XXX? If it is equal to registerEntry.AddedDate, might as well use it and simplify the whole thing
var paymentReceivedAmt = (from registerEntry in RegisterEntries
where registerEntry.TransactionType != null
&& registerEntry.TransactionType.Id.SubsystemCode == "A"
&& registerEntry.Receipt != null
&& registerEntry.PostDate != null
&& registerEntry.PostDate >= lastInvoicedDate
select registerEntry.TransactionAmount.GetValueOrDefault(0)).Sum();

Depending on the LINQ provider you're using you can try the ternary operator:
var paymentReceivedAmt = (from registerEntry in RegisterEntries
where registerEntry.TransactionType != null
&& registerEntry.TransactionType.Id.SubsystemCode == "A"
&& registerEntry.Receipt != null
&& registerEntry.PostDate != null
&& (registerEntry.AddedDate == registerEntry.PostDate
? registerEntry.AddedDate
: registerEntry.PostDate) >= lastInvoicedDate
select registerEntry.TransactionAmount.GetValueOrDefault(0)).Sum();

Null value in linq where clause

I'm having an issue where I want to return results where something matches and I get an error if one of the properties I'm trying to match is null.
if (!string.IsNullOrEmpty(searchString))
{
Infos = Infos.Where(
x =>
x.FirstName.ToLower().Contains(searchString) ||
x.LastName.ToLower().Contains(searchString) ||
x.ContractNum.ToLower().Contains(searchString) ||
x.VIN.ToLower().Contains(searchString) ||
x.Claim.InitiatedBy.ToLower().Contains(searchString)
).ToList();
}
If ContractNum or VIN, for example, are null then it throws an error. I'm not sure how to check if one of these are null inside of a linq query.

You can add explicit null checks:
Infos = Infos.Where(
x =>
(x.FirstName != null && x.FirstName.ToLower().Contains(searchString)) ||
(x.LastName != null && x.LastName.ToLower().Contains(searchString)) ||
(x.ContractNum != null && x.ContractNum.ToLower().Contains(searchString)) ||
(x.VIN != null && x.VIN.ToLower().Contains(searchString)) ||
(x.Claim != null && x.Claim.InitiatedBy != null && x.Claim.InitiatedBy.ToLower().Contains(searchString))
).ToList();

You have multiple options, first is to do an explicit check against null and the other option is to use Null propagation operator.
x.FirstName != null && x.FirstName.ToLower().Contains(searchString)
or
x.FirstName?.ToLower()?.Contains(searchString) == true
But I would suggest you to use IndexOf instead of Contains for case
insensitive comparison.
something like:
x.FirstName?.IndexOf(searchString, StringComparison.CurrentCultureIgnoreCase) >= 0)

Checking the property is null or empty before comparing it it's the only way I know
if (!string.IsNullOrEmpty(searchString))
{
Infos = Infos.Where(
x =>
(!String.IsNullOrEmpty(x.FirstName) && x.FirstName.ToLowerInvariant().Contains(searchString)) ||
(!String.IsNullOrEmpty(x.LastName) && x.LastName.ToLowerInvariant().Contains(searchString)) ||
(!String.IsNullOrEmpty(x.ContractNum) && x.ContractNum.ToLowerInvariant().Contains(searchString)) ||
(!String.IsNullOrEmpty(x.VIN) && x.VIN.ToLowerInvariant().Contains(searchString)) ||
(x.Claim != null && !String.IsNullOrEmpty(x.Claim.InitiatedBy) && x.Claim.InitiatedBy.ToLowerInvariant().Contains(searchString))
).ToList();
}
EXTRA: I added a check on the Claim property to make sure it's not null when looking at InitiatedBy
EXTRA 2: Using the build in function IsNullOrEmpty to compare string to "" and nullso the code is clearer.
Extra 3: Used of ToLowerInvariant (https://msdn.microsoft.com/en-us/library/system.string.tolowerinvariant(v=vs.110).aspx) so the lowering action will act the same no matter of the culture.

You could use ?? to replace it with a acceptable value.
(x.ContractNum??"").ToLower()

I would use the null conditional operator ?, this will however, return a nullable bool? so you will need to handle that appropriately.
Some examples on how to do this:
x?.FirstName?.ToLower().Contains(searchString) == true;
x?.FirstName?.ToLower().Contains(searchString) ?? false;

An alternative method to keep the comparison logic in one place to use a sub collection of the properties and check on those:
Infos = Infos.Where(i=>
new[] {i.FirstName,i.LastName,i.ContractNum /*etc*/}
.Any(w=> w?.ToLower().Contains(searchString) ?? false))
.ToList();
(It does read out all properties, but that shouldn't cost much performance and gains much maintainability )

Inline If statement - short-circuiting

As I understand and read you can use short circuiting in if statement (&& or ||) in order for second condition not to fire. and if you want both condition to fire you would use single operands (& or |).
So say if I have inline if statement as below :
var test = (MyObject != null || string.IsNullOrEmpty(MyObject.Property)) ? string.Empty : MyObject.Property;
This will throw object reference error if MyObject is null, which in my opinion should not as I am using short circuiting. Can someone please explain this.

You're using the wrong condition. This part:
MyObject != null || string.IsNullOrEmpty(MyObject.Property)
should be:
MyObject == null || string.IsNullOrEmpty(MyObject.Property)
The RHS of an || only executes if the left hand is false. You want it to only execute if MyObject is not null.
EDIT: If you really want the MyObject != null part, you could change the whole thing to:
var test = MyObject != null && !string.IsNullOrEmpty(MyObject.Property)
? MyObject.Property : "";
Note the reversal of the 2nd and 3rd operands of the conditional operator too though.

You should have an == not an !=
var test = (MyObject == null || string.IsNullOrEmpty(MyObject.Property) ? string.Empty : MyObject.Property

Try this:
var test = (MyObject == null || string.IsNullOrEmpty(MyObject.Property)
? string.Empty : MyObject.Property

MyObject != null || string.IsNullOrEmpty(MyObject.Property)
Here you say.
If my object is not null.
or
string.IsNullOrEmpty(MyObject.Property)
Which means that if MyObject is null he will try to execute the second part.
MyObject == null || string.IsNullOrEmpty(MyObject.Property)
This won't throw null exception

That happens because MyObject is null and therefore the first condition is false so the second part must be evaluated to know the whole of the condition. Change the line to this:
MyObject != null && string.IsNullOrEmpty(MyObject.Property)

You should prefer readability instead of line-count, e.g.:
string prop = string.Empty;
if(MyObject != null && MyObject.Property != null)
prop = MyObject.Property;
(the reason for your exception was already explained in other answers)

Using ternary operator to check if null - null reference is not the same as null

I am attempting to use the ternary operator to check if a value is null and return one expression or the other. What I'm experiencing when incorporating this into a LINQ expression is that the Transact-SQL translation of the LINQ expression attempts to do a "column = null" rather than a "column IS NULL". I have reason to believe that this is because I'm doing the following:
mappedColumnName == (myVar == null ? null : myOtherVar)
Since it translates the following to columnName IS NULL in Transact-SQL:
mappedColumnName == null
Does anyone have any experience with this? I'd very much like to get this to work.
The entire LINQ expression:
(from MenuItem in menuContext.Menus
where MenuItem.IsSysAdmin == (ClientID == 1 ? true : false)
&& MenuItem.IsActive == true
&& MenuItem.ParentMenuCode == (ActiveSubMenu==null?null:ActiveMenu)
&& MenuItem.ClientID == (UseClientMenu ? ClientID : 0)
&& MenuItem.EmployeeID == (UseEmployeeMenu ? EmployeeID : 0)
orderby MenuItem.SortOrder, MenuItem.MenuName
select MenuItem);

Why don't you use
mappedColumnName == (myVar == null ? DBNull.Value: myOtherVar)
instead?

I've never actually attempted to use ternary operators in linq although you could write it as:
(
(myVar == null && mappedColumnName == null ) ||
(myVar != null && mappedColumnNmae == myOtherVar)
)

try this
(from MenuItem in menuContext.Menus
where MenuItem.IsSysAdmin == ((ClientID == 1 )? true : false)
&& MenuItem.IsActive == true
&& MenuItem.ParentMenuCode == ( (ActiveSubMenu==null) ?null:ActiveMenu)
&& MenuItem.ClientID == (UseClientMenu ? ClientID : 0)
&& MenuItem.EmployeeID == (UseEmployeeMenu ? EmployeeID : 0)
orderby MenuItem.SortOrder, MenuItem.MenuName
select MenuItem);

LINQ to SQL and Null strings, how do I use Contains?

Here is the query
from a in this._addresses
where a.Street.Contains(street) || a.StreetAdditional.Contains(streetAdditional)
select a).ToList<Address>()
if both properties in the where clause have values this works fine, but if for example, a.StreetAdditional is null (Most of the times), I will get a null reference exception.
Is there a work around this?

I'd use the null-coalescing operator...
(from a in this._addresses
where (a.Street ?? "").Contains(street) || (a.StreetAdditional ?? "").Contains(streetAdditional)
select a).ToList<Address>()

The most obvious one:
from a in this._addresses
where (a.Street != null && a.Street.Contains(street)) || (a.StreetAdditional != null && a.StreetAdditional.Contains(streetAdditional))
select a).ToList<Address>()
Alternatively you could write an extension method for Contains that accepts a null argument without error. Some might say that it is not so pretty to have such a method, because it looks like a normal method call, but is allowed for null values (thereby setting aside normal null-checking practices).

You must check first if StreetAdditional is null.
Try
where a.Street.Contains(street) || ((a != null) && a.StreetAdditional.Contains(streetAdditional))
This works because && is a shortcut-operator and if a != null yields false, the second expression with the null-value won't be evaluated since the result will be false anyway.

I would create an extension method to return an empty sequence if null and then call contains method.
public static IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> pSeq)
{
return pSeq ?? Enumerable.Empty<T>();
}
from a in this._addresses
where a.Street.Contains(street) ||
a.StreetAdditional.EmptyIfNull().Contains(streetAdditional)
select a).ToList<Address>()

I don't think SqlServer gave you a null exception. If it did, then this code is clearly not running though LinqToSql (as you've tagged the question).
string.Contains would be translated to sql's like, which has no trouble at all with null values.

Check to make sure that the properties are not null
from a in this._addresses
where (a.Street != null && a.Street.Contains(street)) ||
(a.StreetAdditional != null && a.StreetAdditional.Contains(streetAdditional))
select a).ToList<Address>()
If the null check is false, then the second clause after the && will not evaluate.

from a in this._addresses
where a.Street.Contains(street) || (a.StreetAdditional != null && a.StreetAdditional.Contains(streetAdditional)
select a).ToList<Address>()

You might want to check to make sure the variables street and streetAdditional are not null. I just ran across the same problem and setting them to an empty string seemed to solve my problem.
street = street ?? "";
streetAdditional = streetAdditional ?? "";
from a in this._addresses
where a.Street.Contains(street) || a.StreetAdditional.Contains(streetAdditional)
select a).ToList<Address>()

One thing to note is that the null should be evaluated first.
where (**a.Street != null** && a.Street.Contains(street)) || (a.StreetAdditional != null && a.StreetAdditional.Contains(streetAdditional))
select a).ToList<Address>
()