C# 7.2 brings in to parameter modifiers telling the compiler to error if an attempt is made to modify the value within the method.

This is particularly useful for large structs where passing by value would be expensive, but you want to guarantee the method cannot modify the original.

Code

Now

public double CalculateDistance(in Point a, in Point b)
{
    // a and b are passed by reference but cannot be modified
    return Math.Sqrt(Math.Pow(b.X - a.X, 2) + Math.Pow(b.Y - a.Y, 2));
}

Before

public double CalculateDistance(Point a, Point b)
{
    // a and b are full copies of the structs
    return Math.Sqrt(Math.Pow(b.X - a.X, 2) + Math.Pow(b.Y - a.Y, 2));
}

Notes

WARNING

in parameters are superseded by the ref readonly ^{C# 12.0} which guarantees no copying of the value.

More information

• What's new in C# 7.2
• Proposal
• in vs ref readonly @Stack Overflow
• The in modifier and readonly structs