Any more improvements here are to do with coding quirks. Leetcode's C# Timelimits are too tight.

```
public class Solution {
public int NumberOfArithmeticSlices(int[] A) {
int counts = 0;
if (A.Length == 0)
{
return counts;
}
Dictionary<int, int>[] dp = new Dictionary<int, int>[A.Length];
dp[0] = new Dictionary<int, int>();
for(int i = 1; i < A.Length; i++)
{
dp[i] = new Dictionary<int, int>();
for(int j = i-1; j >= 0; j--)
{
long diff = (long)A[i] - (long)A[j];
if (diff <= Int32.MinValue || diff > Int32.MaxValue) continue;
int df = (int)diff;
if (!dp[i].ContainsKey(df))
{
dp[i][df] = 0;
}
dp[i][df]++;
// Get j's dictionary for the given diff.
if (dp[j].ContainsKey(df))
{
var jsDiffCounts = dp[j][df];
dp[i][df] += jsDiffCounts;
counts += dp[j][df];
}
}
}
return counts;
}
}
```