Lambda Expression +IEnumerable+ C#+ Where+Contains

In C#, I have an IEnumerable<Data> named items And Data is a class that contains 3 string Attributes.

I want to get all items that attribute1 contains a string

public IEnumerable<Data> getDataFiltered(IEnumerable<Data> items,string Filter)
{
    return items.Where(item=>item.Attribute1.Contains(Filter));
}

It throws an exception, however when I use开发者_开发百科:

public IEnumerable<Data> getDataFiltered(IEnumerable<Data> items,string Filter)
{
    return items.Where(item=>item.Attribute1==Filter);
}

It works.

Did I miss something?

You should also actively filter out the results that have a null value for Attribute1

return items.Where(i => !String.IsNullOrEmpty(i.Attribute1) && i.Attribute1.Contains(filter));

When you create a variable of a type, unless you specify a value, it will have the default value for the type. There are two types in .NET reference types and value types. The default value for references types (anything created as a class) is null. A string is an example of a reference type.

public class Data
{
    public string Attribute1 { get; set; }
    public string Attribute2 { get; set; }
    public string Attribute3 { get; set; }
}

The code above, all three attribute values are not initialized and have a value of NULL. To assign a value to the three attributes when create a new instance of your object with 'new Data()' you can create a constructor that assigns a value.

public Data()
{
    Attribute1 = Attribute2 = Attribute3 = String.Empty();
}

Now all attributes will have the empty string value. Meaning they are initialized but don't have a value.

Value types (anything defined as a struct) can not be NULL. Examples of this are DateTime, int, double, decimal, etc. The default value for number types is 0. The default value for DateTime is DateTime.MinValue.

I assume the value of Attribute1 was null then calling .Contains will crash and == will not.

Some of your Attribute1s are probably null.

You can't call Contains on null.

Consdider this:

public IEnumerable<Data> getDataFiltered(IEnumerable<Data> items, string Filter){

    return items.Where(item => item != null && item.Contains(Filter));
}

I added the nullity checks to make sure that Contains isn't called on null reference. String is a reference type which means that it can occasionaly be null.

If you want to check for the exact value use == if you want to check for the occurence of a string use either of the following.

For a case sensitive approach the following :-

items.Where(item => (item ?? "").Contains(Filter));

?? is the coalesc operator in c# just like sql's ie the first non value

Contains however is case sensitive so if this is not desired then the following might be what you want.

items.Where(item => -1 != (item ?? "").IndexOf(Filter, StringComparison .InvariantCultureIgnoreCase));