Linq Dynamic Query Issue - Operator '&&' incompatible with operand types 'String' and 'Boolean'

I have a method which generates from my query.

It looks like this:

private static string PrepareWhereClause(string[] columns)
{
    var query = $"({string.Join(" OR ", columns?.Select(c => $"{c} != null AND {c}.ToLower().Contains(@0)"))})";
    return query;
}

This query is used on Where extension from Linq dynamic like this:

public static IQueryable<T> Search<T>(this IQueryable<T> queryable, string phrase, params string[] columns) =>
        queryable?.Where(PrepareWhereClause(columns), phrase.ToLower());

My object that is used in the query is:

public class ResponsibilitiesWebDto
{
    public Guid Id { get; set; }
    public DictionaryBaseDto ResponsibilityType { get; set; }
    public UserForDetailedDto Employee { get; set; }
    public SupplierForDetailedDto Supplier { get; set; }
}

On argument for PrepereWhereClause are field of type string which are inside in ResponsibilityType, Employee and Supplier.

Query that is generated by PrepereWhereClause is:

(ResponsibilityType.Name != null AND ResponsibilityType.Name.ToLower().Contains(@0) OR Employee.EmployeeId != null AND Employee.EmployeeId.ToLower().Contains(@0) OR Employee.Name != null AND Employee.Name.ToLower().Contains(@0) OR Employee.LastName != null AND Employee.LastName.ToLower().Contains(@0) OR Supplier.AccountNum != null AND Supplier.AccountNum.ToLower().Contains(@0) OR Supplier.Name != null AND Supplier.Name.ToLower().Contains(@0))

All is ok when ALL child elements are not null.

Problem is when some of them are null. I looking to this post on SO Dynamic Linq Core OrderBy Nullable Child Property and on issue on github and I try use but it doesn't work :(

I modified my query to this:

private static string PrepareWhereClause(string[] columns)
{
    var query = $"({string.Join(" OR ", columns?.Select(c => $"{c} != null AND {c}.ToLower().Contains(@0)"))})";
    var query1 = $"({string.Join(" OR ", columns?.Select(c => "((" + ConvertToNullableNested(c) + $") AND {c}.ToLower().Contains(@0))"))})";
    var query2 = $"({string.Join(" OR ", columns?.Select(c => $"np({c} != null AND {c}.ToLower().Contains(@0)"))})";
    var query3 = $"({string.Join(" OR ", columns?.Select(c => $"(np({c}) AND {c}.ToLower().Contains(@0))"))})";
    var query4 = $"({string.Join(" OR ", columns?.Select(c => $"(np({c}) && {c}.ToLower().Contains(@0))"))})";

    return query4;
}

New query now is:

((np(ResponsibilityType.Name) && ResponsibilityType.Name.ToLower().Contains(@0)) OR (np(Employee.EmployeeId) && Employee.EmployeeId.ToLower().Contains(@0)) OR (np(Employee.Name) && Employee.Name.ToLower().Contains(@0)) OR (np(Employee.LastName) && Employee.LastName.ToLower().Contains(@0)) OR (np(Supplier.AccountNum) && Supplier.AccountNum.ToLower().Contains(@0)) OR (np(Supplier.Name) && Supplier.Name.ToLower().Contains(@0)))

But I always get this error:

Operator '&&' incompatible with operand types 'String' and 'Boolean'

I try change && to AND but nothing change. What am I doing wrong?

I change query like @orxanmuv write but I still get an error.

Query for it:

(((np(ResponsibilityType.Name) == null or np(ResponsibilityType.Name)) AND ResponsibilityType.Name.ToLower().Contains(@0)) OR ((np(Employee.EmployeeId) == null or np(Employee.EmployeeId)) AND Employee.EmployeeId.ToLower().Contains(@0)) OR ((np(Employee.Name) == null or np(Employee.Name)) AND Employee.Name.ToLower().Contains(@0)) OR ((np(Employee.LastName) == null or np(Employee.LastName)) AND Employee.LastName.ToLower().Contains(@0)) OR ((np(Supplier.AccountNum) == null or np(Supplier.AccountNum)) AND Supplier.AccountNum.ToLower().Contains(@0)) OR ((np(Supplier.Name) == null or np(Supplier.Name)) AND Supplier.Name.ToLower().Contains(@0)))

Error:

Operator 'or' incompatible with operand types 'Boolean' and 'String'