LINQ Dynamic QUERY to produce 'where AssignedUser is null'

Question

I am trying build dynamic expression query to get only rows which has null in the column

where AssignedUser is null

, below is my code, but it is not doing what i expected. Can any one spread the light on the issue please?

private Expression<Func<VwAssignmentActivities, bool>> GetIsNullExpressionEquals<T>(string propName, T value)
{
    var item = Expression.Parameter(typeof(VwAssignmentActivities), "item");
    var prop = Expression.Convert(Expression.Property(item, propName), value.GetType());

    Expression body = Expression.Equal(prop, Expression.Constant(null, prop.Type));

    return Expression.Lambda<Func<VwAssignmentActivities, bool>>(body, item);
}

Any help appreciated

Answer 1

The error you are getting is because you have a value in your Nullable. The get type returns Int32 even though the variable is a Nullable. Then you are trying to convert the null to int.

Assuming you only care about finding null values, I would do something like this

public Type GetNullable(Type type)
{
    if (type == typeof(Nullable<>))
        return  type.GetType();
        
    if(type == typeof(int))
        type = typeof(int?);
    else if(type == typeof(bool))
        type = typeof(bool?);
    else if(type == typeof(float))
        type = typeof(float?);      
    // etc. you  will have to build out every type you want.
    
    return type;
}

public Expression<Func<VwAssignmentActivities, bool>> GetIsNullExpressionEquals(
    string propName, Type value)
{       
    var item = Expression.Parameter(typeof(VwAssignmentActivities), "item");
    var prop = Expression.Convert(Expression.Property(item, propName), 
        GetNullable(value));
    
    Expression body = Expression.Equal(prop, Expression.Constant(null, 
       prop.Type));
    
    return Expression.Lambda<Func<VwAssignmentActivities, bool>>(body, item);
}