private static BlockSyntax CreateAddSyntax(IList <PropertyDeclarationSyntax> properties)
{
// var hash = new HashCode();
var declaration = VariableDeclaration.Create("hash", nameof(HashCode));
var addExpressions = properties.Select(CreateAddInvocation).Cast <StatementSyntax>().ToList();
// return hash.ToHashCode();
var returnStatement = ReturnStatement(
InvocationExpression(AccessMemberSyntax.Create("hash.ToHashCode")));
var statements = new StatementSyntax[addExpressions.Count + 2];
statements[0] = declaration;
addExpressions.CopyTo(statements, 1);
statements[^ 1] = returnStatement;