C# 7.0 introduced out variables and pattern matching which allowed declaring variables inline. However, these expression variables were not permitted in field initializers, constructor initializers (this() / base()), or LINQ query clauses.

C# 7.3 removes these restrictions, allowing expression variables to be used in these additional contexts.

Code

Now

class Order
{
    // Out variable in field initializer
    bool valid = int.TryParse(Config.MaxItems, out var maxItems);

    // Expression variable in constructor initializer
    public Order(string input) : base(int.TryParse(input, out var id) ? id : 0)
    {
    }
}

// Pattern variable in LINQ query
var results = from s in strings
              where int.TryParse(s, out var n) && n > 10
              select n;

Before

class Order
{
    int maxItems;
    bool valid;

    public Order()
    {
        valid = int.TryParse(Config.MaxItems, out maxItems);
    }
}

// Had to filter and parse separately
var results = strings
    .Where(s => int.TryParse(s, out _))
    .Select(s => int.Parse(s))
    .Where(n => n > 10);

More information

• Specification