複数のラムダ式から式木を構築する ref: http://qiita.com/items/438b845154c6c21fcba5

first.cs

Expression<Func<int, int, bool>> expr1 = (x, y) => x * y < 20;
Expression<Func<int, int, bool>> expr2 = (x, y) => x + y == 8;

Expression<Func<int, int, bool>> expr = CombineLambda(expr1, expr2);
Console.WriteLine(expr.Compile()(3, 5)); // True

public static Expression<TDelegate> CombineLambda<TDelegate>(Expression<TDelegate> left, Expression<TDelegate> right)
{
    // TODO: ここを埋めたい！
    throw new NotImplementedException();
}

test1.cs

public static Expression<TDelegate> CombineLambda<TDelegate>(Expression<TDelegate> left, Expression<TDelegate> right)
{
    return Expression.AndAlso(left, right);
}

test2.cs

public static Expression<TDelegate> CombineLambda<TDelegate>(Expression<TDelegate> left, Expression<TDelegate> right)
{
    return Expression.Lambda<TDelegate>(Expression.AndAlso(left, right));
}

test3.cs

public static Expression<TDelegate> CombineLambda<TDelegate>(Expression<TDelegate> left, Expression<TDelegate> right)
{
    return Expression.Lambda<TDelegate>(Expression.AndAlso(left, right));
}

test4.cs

public static Expression<TDelegate> CombineLambda<TDelegate>(Expression<TDelegate> left, Expression<TDelegate> right)
{
    return Expression.Lambda<TDelegate>(Expression.AndAlso(left.Body, right.Body), left.Parameters);
}

test5.cs

public static Expression<TDelegate> CombineLambda<TDelegate>(Expression<TDelegate> left, Expression<TDelegate> right)
{
    var visitor = new ParameterVisitor(left.Parameters);
    return Expression.Lambda<TDelegate>(Expression.AndAlso(left.Body, visitor.Visit(right.Body)), visitor.Parameters);
}

visitor.cs

public class ParameterVisitor
    : ExpressionVisitor
{
    private readonly IDictionary<Tuple<Type, string>, ParameterExpression> _parameters;

    public ICollection<ParameterExpression> Parameters
    {
        get
        {
            return this._parameters.Values;
        }
    }

    public ParameterVisitor(IEnumerable<ParameterExpression> parameters)
    {
        this._parameters = parameters.ToDictionary(p => Tuple.Create(p.Type, p.Name));
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        var key = Tuple.Create(node.Type, node.Name);
        return this._parameters.ContainsKey(key)
            ? this._parameters[key]
            : node;
    }
}